Measurement Techniques for

Solar Cells, Annual Report

September 15,1 977 to

December 14, 1978

D. E. Sawyer, H. K. Kessler,

I and H. A. Schafft

i

i

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

July 1980

100

U56

80-2027

1980

C.2

Prepared for Department of Energy

Division of Distributed Solar Technology

Advanced Materials R&D Branch

Under Task Order A054-SE of

Interagency Agreement EA-77-01 -6010
tu.\
NBSIR 80-2027

MEASUREMENT TECHNIQUES FOR

SOLAR CELLS, ANNUAL REPORT

SEPTEMBER 15, 1977 TO

DECEMBER 14, 1978

AnOMAX. BUREAU or standards

UBRARY

5£p 1 9 1980

D. E. Sawyer, H. K. Kessler,

and H. A. Schafft

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

July 1980

Prepared for Department of Energy

Division of Distributed Solar Technology

Advanced Materials R&D Branch

Under Task Order A054-SE of

Interagency Agreement EA-77-01-6010

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Measurement Techniques for Solar CellS/ Annual Report

September 15/ 1977 to December 14, 1978

D. E. Sawyer, H. K. Kessler, emd H. A. Schafft

Table of Contents

Page

Preface vii

1. Executive Summary 1

2. Applications of the Laser Flying-Spot Scanner to Solar Cells ... 4

2. 1 Overview of Some Cell Problems 4

2.2 Spatial Variation of Cell Response 4

3. Early Laser Scanning Work Using Light Modulated at Microwave

Frequencies 6

4. Development of Solar Cell Device and Material Measurement

Techniques 9

4. 1 Cell Scanning Technique Employing Light and Current Source

Cell Biasing 9

4.1.1 Description of Technique 9

4.1.2 Cell Scanning Results 15

4. 1.2.1 Emitter Metallization Breaks 15

4. 1.2.2 Metallization Not in Ohmic Contact With the

Emitter Region 15

4. 1.2.3 Cell Crack E)etection 17

4. 1.2.4 Other Scanning Results ... 21

4. 1.2.5 Further Applications of the Laser Scanner . . 21

4.2 Equipment for Light-Biasing Cells 22

4.3 Laser Flying-Spot Scanner Modifications 29

4.4 Ancilleury Apparatus 29

4.4.1 Solar Cell Impedance-Matching Network and Preamplifier 29

4.4.2 Solar Cell Cleunping and Rocking Stage 32

4.5 Reference Structure Arrays 32

4.6 Mathematical Analysis 34

4.6.1 Laser Scanning Using Microwave-Modulated Light .... 34

4.6.2 Laser Scanning of Forward-Biased Cells 37

4.6.3 Mathematical Modeling Work Performed Under Subcontract

with the University of Southern California 38

4. 6. 3.1 Introduction 38

4. 6. 3. 2 Basic Equations . 38

4. 6. 3. 3 Low Intensity, Modulated Illumination .... 41

4. 6. 3. 4 Fault-Free Cells Having Uniform Attenuation

Lengths ..... 42

4. 6. 3. 5 Fault-Free Cells Illuminated by a Line-Shaped

Light Beam 43

4. 6. 3. 6 Fault-Free Cells Illuminated by a Point of

Light 44

iii
Page

4. 6. 3. 7 Output of Faulty Cells Having Noncontacting

Electrode Sections 44

5. Workshops 46

5.1 Stability of (Thin Film) Solar Cells and Materials 46

5.2 Photovoltaic Material and Device Measurements ... 46

6. Consultation and Liaison Activities 47

7. References 48

Appendix A Laser Scanning of Active Integrated Circuits and Discrete

Semiconductor Devices 49

Appendix B Stability of (Thin Film) Solar Cells and Materials - Workshop Program 55

Appendix C Photovoltaics Material and Device Measurements Workshop - Preliminary Plans 59

Appendix D Publications and Talks 61

LIST OF FIGURES

1. Silicon p-n junction diode photoresponse. (a) Photomicrograph of

diode array; the diode scanned is the central one in the array.

(b) Photoresponse to unmodulated 0.633-ym light and with the diode reverse biased 5 V. (c) Photoresponse for the same conditions ex- cept that the screen now displays the diode response to the light

component modulated at 500 MHz. (d) Same as (c) except the modulation frequency is 1.0 GHz 7

2. A model relating the diode scanning results shown in figure 1 to

the diode geometry and electrical characteristics. (a) Crosssectional view of diode (not to scale). (b) Equivalent circuit

of diode. (c) Photoresponse to 0.633-ym unmodulated light and

0.633-ym light modulated at 0.5 and 1.0 GHz 8

3.

A cross section, not to scale, of an idealized solar cell con- nected to a load Z^, showing the portion between a contiguous

pair of parallel metallization stripes 11

4.

The one-dimensional electrical representation of figure 3 for

the case of laser scanning with a line of light. The cell

emitter is represented by the continuous resistive element, and

the small-signal voltage-current ratios for the elemental diodes

making up the cell between the stripes are represented by the

discrete resistors shown connected with dotted lines emd the line

of light by an elemental current source 11

iv
Page

5. The variation of cell output voltage with scanning light position between parallel metallization stripes for a representative value of (35-/2) of 1.30 11

6. Idealized solar cell; (a) The I-V characteristics of the cell

under both dark and illuminated conditions. (b) The eqxiivalent

circuit where the strength of the current generator is proportional to the illuminating light intensity 12

7. Air-mass-one insolation levels versus desired scanningsensitivity ratios for various combinations of cell stripe spacing (£) and emitter sheet resistance (p^) for a junction silicon

solar cell 14

8. A 2- by 2-cm (0.8- by 0.8-in.) silicon "^ace" cell: (a) Photograph of the cell. (b) Sketch of cell with metallization breaks

shown by dots. (c) Scanning photoresponse using the y-axis display mode and with the cell unbiased. (d) Scanning photoresponse

using the y-axis display mode and with a cell bias of 486 mA. (e)

Scanning photoresponse using the intensity-modulation display mode

and with a cell bias of 486 mA 16

9. A 3.3- by 3.3-cm (1.3- by 1.3-in.) polycrystalline silicon solar

cell; (a) Photograph of the cell. (b) Scanning photoresponse

using the intensity-modulation display mode and with the cell unbiased. (c) Scanning photoresponse using the y-axis display mode

and with the cell unbiased. (d) Scanning photoresponse using the

intensity-modulation display and with a cell bias of 100 mA. (e)

Scanning photoresponse using the y-axis display mode and with a cell bias oflOOmA 18

10. The results of scanning two small portions of a cracked 100-mm

(4-in.) diameter silicon cell using the y-axis display mode. (a)

Scanning photoresponse of the first portion with the cell un- biased. (b) Scanning photoresponse of the same portion as shown

in (a) but with a cell bias of 0.9 A. (c) Scanning photoresponse

of the second portion with the cell unbiased. (d) Scanning re- sponse of the Scune portion as shown in (c) but with a cell bias

of 0.9 A. The cell crack location is indicated by arrows in (b)

and (d) 20

1 1 . The high-intensity light source with the top cover removed .... 24

12. An overall view of the high-intensity light source including

fiber-optic light pipes 25

13. The illuminator being used during the laser scanning of a solar cell 26

14. A close-up view showing the means used to adjust and clamp the

light-emerging ends of the fiber-optic light pipes 27

V
Page

15. A photograph taken near the end of the reporting period of the

laser scanner and some of the emcillary equipment described in

this report. One of the 5.63-cm (2.25-in.) diameter cells in

a 36-cell module is being scanned 28

16. Schematic diagram of the solar cell coupling network. The criteria for the selection of the components and the component part

values are stated in the text 31

17.

Solar cell clamping and rocking unit attached to the microscope

cross-slide stage. Cell clamping is performed with a vacuxim

hold-down chuck magnetically held to the top plate of the rocking mechanism. Rocking (the orientation of the vertical axis) is

controlled by micrometers. This photograph was made with the scanner in operation, and the scanning light raster is approximately in the middle of the cell 33

18.

Representation of the two-dimensional parasitic resistancecapacitance nature of a reverse -biased solar cell 35

19. One-dimensional cross-section through the equivalent circuit

of a solar cell portion illuminated by a modulated light beam

at X = x-j. Output is ac-shorted by a large capacitive load ... 39

20. Sketch of a solar cell showing finger electrodes spaced by distance s and an illuminated spot at (x-j, 0) 40

21.

Element of the distributed network shown in figure 19 indicating the notation used in section 4.6.3 40

vi
PREFACE

This work was conducted as a part of the Semiconductor Technology Program of

the National Bureau of Standards (NBS). This program serves to focus NBS re- search to enhance the performance, interchangeability, and reliability of integrated circuits and other semiconductor devices including solar cells

through improvements in measurement technology for use in specifying materials and devices in national and international commerce and for use industry in controlling device fabrication processes. This research leads to

carefully evaluated and well-documented test procedures and associated technology. Special emphasis is placed on the dissemination of the results of

the research to the appropriate technical community. Application of these

results by industry will contribute to hi^er yields, lower cost, and higher

relicJaility of semiconductor devices. Improved measurement technology also

leads to greater econon^ in government procurement by providing a common basis for the purchase specifications of government agencies and, in addition,

provides a basis for controlled improvements in fabrication processes and in

essential device characteristics.

The work reported herein was supported by the Division of Distributed Solar

Technology of the Department of Energy (DOE) under DOE Task Order A054-SE of

Interagency Agreement EA- 77 -A-0 1-6010. The task order is monitored by the

SERI Photovoltaics Program Office (PVPO), Dr. Donald L. Feucht, Chief. The

NBS point of contact for information on the various task elements of this

project is Harry A. Schafft of the Electron Devices Division in the NBS Center for Electronics and Electrical Engineering.

The objectives of the work are to support the PVPO photovoltaic effort in the

following ways;

1 . By developing solar cell device and material measurement techniques

using the laser flying-spot scanner originally developed at NBS for

use on integrated circuits and discrete transistors, and

2. By assisting DOE in organizing and hosting appropriate workshops and

symposia, and by providing general consultation and liaison services.

Disclaimer

Certain commercial equipment, instruments, or materials are identified in

this report in order to adequately specify the ejq>erimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment

identified is necessarily the best available for the purpose.

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Measurement Techniques for Solar Cells, Annual Report

September 15, 1977 to December 14, 1978

D. E. Sawyer, H. K. Kessler, and H. A. Schafft

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

1 . Executive Summary

This report covers research performed in the period September 15,

1977 to December 14, 1978 under the program Solar Cell Measurement

Technique Development and Other Services by the Electron Devices

Division of the National Bureau of Standards. The objectives of

this project are to provide support to the Department of Energy

(DOE) thin-film photovoltaic effort in the following ways: (1) by

developing solar cell device and material measurement techniques

using the NBS-developed laser flying-spot scanner and (2) by as- sisting the DOE in organizing and hosting appropriate workshops and

symposia and providing general consultation and liaison services.

In this annual report, many applications of the laser scanner are

described, and others are proposed. The laser scanner itself is

described in Appendix A. The applications described include detecting cell cracks, determining metallization "breaks," and locating metallization regions making poor ohmic contact with the underlying emitter surface. Preliminary work on gallium arsenide metalinsulator-semiconductor (GciAs MIS) cells indicates that the scanner

can identify portions of a cell which are less effective than others due to "island formation," i.e., break-up of the thin, optically transparent metal film, which may occur in these structures.

The scanner can also provide information useful to the cell designer.

At the outset of the work it was thought that information such as that enumerated above could only be obtained by scanning with light

modulated at microwave frequencies, but a technique much simpler

than this was conceived and successfully developed. It employs

forward biasing of the cell during scanning. Forward bias may be

provided by use of an illumination source, by an externally supplied current source, or by a combination of the two. The scanning

technique is nondaraaging; it requires no electrical contacts to the

cell other than those already present, and it can be used on encapsulated or unencapsulated cells in almost any laboratory or test

envi ronment.

The case of scanning between parallel emitter metallization stripes

using a line of light was analyzed for two cases: (1) a reverse- biased cell illuminated with the light modulated at microwave frequencies and (2) a forward-biased cell illuminated with unmodulated

light. The mathematics is similar for both situations. For the

1
latter, the maximum-to-minimum cell response signal ratio* depends only on the emitter sheet resistance, the stripe separation,

and the cell conductance per unit area; the last can be varied by

adjusting the cell current. The analysis predicts that the emitter

sheet resistance can be obtained by fitting the analytical value

for the maximum-to-minimum ratio to the e?q)erimentally observed ratio. Drs. K. Lehovec and A. Fedotowsky, working under subcontract

at the University of Southern California, extended the mathematical

analysis and cell modeling effort to include the use of a scanning

light spot on more realistic cell geometries than the one- dimensional case described above.

The results of the in-house device modeling work and mathematical

analysis outlined above were used to develop the specifications re- quired of a high-intensity light source for cell biasing during

laser scanning. This source was subsequently constructed and suc- cessfully employed.

Before this program began, the laser scanner had been used primarily for ICs and other transistor- like devices. To obtain optimum

conditions for scanning solar cells, several modifications were made to the scanning apparatus, and ancillary equipment (including

the light source mentioned above) was designed and constructed.

Optical elements were acquired to allow the raster scanning region

to be increased from a previous maximum area value of 3 mm (0.12

in.) square to an area as large as 100 mm (4 in.) square. A vacuum hold-down and rocking stage were fabricated (the rocking feature is

needed when one desires ref lected- light information). A simple but

effective jig was designed and constructed to permit each of the 36

5.7-cm (2.25-in.) diameter cells in a 25.4-cm (10-in.) by 61-cm

(24-in.) module to be scanned while biased to reveal cell defects.

A means was developed to couple forward-biased, and consequently

low- impedance, scanned cells to the display electronics without degrading the cell's inherent slgnal-to-noise ratio and video information content.

The design of a mask set for a reference structure array (RSA) was

completed. The results of measurements on these RSAs will be used

to verify the validity of using the mathematical modeling work, including the presence of various types of cell defects, to quantify

the results of laser scanning experiments. The objective of this

verification is to put the laser scanning techniques for solar cells

on a firm analytical basis. The RSA consists of four solar cells,

each surrounded by test patterns to give a mean value of, and the

spatial variation in, the following: cell cind material properties

and parameters (including emitter and metallization sheet resistance), metal-to-emitter contact resistance, substrate resistivity,

and carrier density in the base as a function of distance below the

emitter-base junction. The first solar cell contains no intentional defects; the second contains emitter metallization stripes with

*Response adjacent to metallization to response at mid-distance to

next parallel metallization stripe.

2
"open" regions of various controlled sizes; the third has no opens,

but does have an emitter sheet resistance which varies periodically

in a stripe pattern; and the fourth contains both opens and the

emitter sheet resistance variation described above. The RSAs will

be made from single-crystal silicon at the NBS fabrication facility; only silicon technology offers the fabrication precision re- quired.

A workshop on the Stability of (Thin Film) Solar Cells and Materials was conducted at the National Bureau of Standards on May 1 to

3, 1978 as part of the Department of Energy's National Photovoltaic

Program. The workshop program is included as Appendix B. The

workshop addressed many of the problems and obstacles to achieving

stability and long life of terrestrial solar cell devices using advanced technology. The following three groups of exploratory solar

cell materials and concepts were considered: (1) CU2 S/ [CdZn] S,

Cu-Ternaries/CdS, CdS/InP, and amorphous Si; (2) polycrystalline

Si, MIS, and conducting oxide Si; and (3) polycrystalline and AMOS

GaAs.

During this reporting period, plans were initiated for a workshop

on photovoltaic material and device measurements, scheduled to be

held in the Washington, D.C. area in the spring of 1979. The purpose of the workshop is to accelerate the development of thin-film

solar cells by improving the versatility and reliability of material and device measurement techniques. The preliminary plans for

the workshop are included as Appendix C.

The publications and talks associated with the work conducted in

this reporting period are listed in Appendix D.

3
2. Applications of the Laser Flying-Spot Scanner to Solar Cells

2.1 Overview of Some Cell Problems

The requirements placed on terrestrial solar cells developed for DOE's Photovoltaic Program are unique in that the combination of adequate efficiency,

low cost, and long life is demanded. Satisfying this set of requirements

means that some serious problems have to be faced and overcome. Some of the

cell types that are leading contenders for wide-spread deployment may be made

from poorly understood materials by inadequately controlled and understood

fabrication techniques. History has shown that these inadequacies can some- times lead to periods of cell production during which cell performance is inexplicably poor.

An cLlternate approach to satisfying DOE ' s requirements is to use small

amounts of relatively expensive but well-understood materials such as singlecrystal silicon or gallium arsenide, eind rely on low-cost reflectors or Fresnel lenses to collect and focus the sunlight on the cell with concentration

ratios of up to several thousand. The adequate design of a concentrator

solar cell can be a formidable problem. An appropriate analysis would begin

with a device model suitable for computer simulation in three dimensions

where the device pareuneters values could vary over this space. There are disadvantages to this approach, however. One is that the comprehensiveness

of the model could obscure the cause and effect relationships between device

parameter values chosen and overall cell efficiency. That is, the possible

usefulness of the computer results in predicting what should be done to

improve the cell may be obscured by the many quantities one could vary; it is

not known on an a pyrtori basis which quantities are important and must be

^ecified accurately, and which quantities may safely be neglected for the

sake of sin^licity, to obtain a solution which may lead to better design insight .

A third important class of cells, nonconcentrator types made from singlecrystal materials, is fabricated by techniques which are quite similar to

those developed by the integrated circviit industry. The solar cell emitter

region (that portion above the p-n junction) is quite thin to enhance the

cell response in the blue region of the spectrum, typically less than half a micrometer in the more recent efficient designs. The thinness of the emitter

makes it difficult to fabricate uniform, low-resistance contacts to the emitter without having the metallization short circuit the junction.

The laser flying- spot scanner can be applied to aid in overcoming mciny of the

above problems. Ways in which the scanner can be so used are reported in

this document.

2.2 Spatial Variation of Cell Response

It would be extremely useful, especially for concentrator cells, if one could

measure the spatial uniformity of the cell response over its surface. This

could provide one check of the cell's design and fabrication steps. The scanner has been used in a rather simple manner to do this. It has been used

to reveal those portions of a cell which are not as effective as other parts

in producing load power under illumination. Furthermore, it presents this

4
information in a quantitative manner so as to allow an accurate assessment of

the cell deficiencies. This is a straightforward scanner application. The

technique described in this report can be used for any cell composition; it

provides information badly needed for concentrator cell design and assessment. It can also be used to obtain a better understanding of the behavior

of nonconcentrator cells, both of the conventional p-n junction singlecrystal variety and those made using structures and materials of a more ex- ploratory nature.

The technique itself is very simple. The cell is scanned with a spot of

light while it is uniformly illuminated by a bias light that can be increased

step-wise in intensity. By self-biasing the cell with light, one can simulate operation in the field. For some cell types, one may also provide cell

biasing by means of an externally imposed electrical current source and

achieve equivalent scanning results. Under these conditions, the scanner

display screen presents a map of the small-signal, or incremental, cell re- sponse. By scanning the cell under various loads between zero ohms (the cell

termination for measurement of short-circuit photocurrent) and infinity (the

cell termination for measurement of open-circuit photovoltage ) , one can obtain maps which when compared to each other show the load-to-load variation

of the photoresponse over cell. The maps can further be interpreted to yield

desired information eibout the cell's point-to-point operation. These biasing

techniques, and the scanning results obtained, are discussed in section 4.

Prior to the present reporting period, experiments were made on reversebiased p-n jvinction diodes scanned with light modulated at microwave frequencies to reveal the diode's passive and distributed nature. This work is included in the next section to serve as an introduction to the present, more

convenient, scanning technique. The present technique uses cell dc forward

biasing, and unmodulated scanning light,* to reveal not only the passive

nature, e.g. , emitter sheet resistance, metallization defects, etc., but also

the active nature of the cell.

\

*For purposes of this report, "unmodulated scanning light" may also include

light modulated at frequencies well above those contained in the waveform of

the cell's normal response to scanned unmodulated light.

5
3. Early Laser Scanning Work Using Light Modulated at

Microwave Frequencies

The photomicrograph in figure la shows the silicon chip used for the first

laser scanning observations of emitter lateral (sheet) resistance. There are nine diodes, each about 250 iJm on a side, formed by an n-type planar diffusion into p-type silicon of about 5-fi*cm resistivity to achieve a junction

depth of about 1 vrti. These values of junction depth and bulk resistivity are close to those used for commercial silicon solar cells. Conventional techniques were used to bond the nine-diode chip to a header and to make a wire

bond connection between one of the two ohmic contacts on the central diode

and a header post terminal.

The other photographs in figure 1 are of the scanner display screen i^owing

the photoresponses of the central diode due to its being scanned with laser

light of wavelength 0.633 )jn while a reverse voltage of 5 V is applied to the

diode. The photoresponses are presented as a superposition of an intensity

modulated signal (conventional z-axis mode) and a vertical deflection signal.

Figure 1b is a photograph of the diode's response to unmodulated light. Fig- ure 1c shows the response of the diode to a 500-MHz modulated component su- perimposed on the unmodulated light while the bias conditions are maintained.

Figure Id is the corresponding response to a 1.0-GHz modulation. The gain of

the display screen was adjusted to maintain the same maximum vertical deflection in all three photographs. This maximum occurs when the scanning spot is

located at the edge of the bonded contact. It is seen that the 500-MHz and

1.0 -GHz photoresponses from the portions of the diode away from the bonded

contact decrease as the modulation frequency increases and as the distance

from the contact increases.

Figure 2 can be used to explain the results shown in figure 1 . Figure 2a is

a cross section throucfi the diode, and figure 2b is the corresponding one- dimensional electrical representation of the diode. The diffused emitter region is represented by a distributed resistor and the junction transition re- gion by a distributed capacitor. One would e>q)ect such a network to behave

in the manner observed; the signal to the radio receiver would decrease with

increasing signal frequency and with increasing distance of the laser excitation from the bonded contact, as sketched in figure 2c, because the attenuation due to the distributed resistance-capacitance line increases with these

quantities.

6
c. d.

Figure 1. Silicon p-n junction diode photoresponse. (a) Photomicrograph of

diode array; the diode scanned is the central one in the array. (b) Photoresponse to unmodulated 0.633-ym light and with the diode reverse biased 5 V.

(c) Photoresponse for the same conditions except that the screen now displays

the diode response to the light component modulated at 500 MHz. (d) Same as

(c) except the modulation frequency is 1.0 GHz.

7
a.

Figure 2. A model relating the diode scanning results shown in figure 1 to

the diode geometry and electrical characteristics. (a) Cross-sectional view

of diode (not to scale). (b) Equivalent circuit of diode. (c) Photoresponse

to 0.633-ym unmodulated light and 0.633-ym light modulated at 0.5 and 1.0

GHz.

8
4. Development of Solar Cell Device and Material Measurement Techniques

4. 1 Cell Scanning Technique Employing Light and Current Source Cell Biasing

A new method was conceived to detect and study cell faults, such as regions

of poor metallization, and to determine important cell parameters such as emitter sheet resistance. This method was applied to a variety of cell types

and the basic soundness of the method was confirmed.

The method makes use of a cell's distributed nature when it is forward biased, bias conditions similar to those which exist when a solar cell is ex- posed to sunlight. Unmodulated scanning light is employed. The scanning

light source most often used is a low-power (3-mW) continuous wave heliumneon laser operating at 0.633 ym. Also used is a laser operating at 1.15 ym.

The effect of scanning a solar cell with a light ^ot is to place a signal

current source across the cell barrier, e.g., the p-n junction, at the point

illuminated by the i^ot. The bias may be provided by either exposing the

cell to light, or by connecting the cell to an external electrical current

source, or by a combination of these two means. The results of calculations,

which have been confirmed by measurements, show that the cell currents required to display the desired scanner information are comparable to those

which normally occur within the cell when the cell is exposed to irradiance

levels for which it was designed.

The new method allows one to detect potentially harmful cell cracks with a sensitivity greater than any previously reported method. It also allows one

to locate regions of cell emitter metallization which are either broken or not in good ohmic contact with the underlying emitter. The method is much

easier to employ than the originally envisioned scheme in which a reversebiased cell is illuminated with light modulated at microwave frequencies.

Other attributes of the method are: it is nondamaging, it requires no electrical contacts to the cell other than those already present, and it can be

used on encapsulated or unencapsulated cells in cLLmost any laboratory or test

environment. A preliminary account of the method was given at the IEEE 13th

Photovoltaic Specialists Conference [1], and a more complete description was made at the 1978 Report Session of the Advanced Materials R&D Branch [2].

4.1.1 Description of Technique

The low-frequency small -signal equivalent circuit of a biased and scanned

cell is a three-dimensional resistive array. The scanning light spot is represented by a current generator moving within the array as the i^ot scans the

cell. The array components are normally made up of the cell emitter sheet

resistance and the distributed resistance for the p-n junction. This distributed resistance is the local slope of the voltage-current curve for each

incremental cell curea. A simplified, one-dimensional analysis of this array

is presented in section 4.6.2. The highlights are given in this section.

The analysis proceeds in a manner similar to the one for light modulated at

microwave frequencies presented in section 4.6.1; in the present case, the

local voltage-current slope associated with the p-n junction plays a role

similar to the distributed transition-region junction capacitance for the

9
case in which the light is modulated at microwave frequencies. Implementation of the newer technique is easier because high-frequency equipment and

techniques are not needed.

Figure 3 is an end-on view, not to scale, of an idealized solar cell showing

the portion between a contiguous pair of (parallel) metallization stripes,

and figure 4 is its one-dimensional electrical representation for the case of

scanning with a line of light. This type of scanning will probably not be

performed in the program, but this case was used for analysis in section

4.6.2 because the solution can be obtained as a closed-form expression which

may be exeunined to anticipate the important results for more realistic twodimensional cases. In figure 4, the emitter is represented by the continuous

resistive element, and the small-signal voltage-current ratios for the elemental diodes making up the cell between the stripes are represented by the

discrete resistors connected with dotted lines. In the absence of significant shunting by the latter, the photocurrent will find its way to the load

through one path or another. Thus, while the load voltage (the scanner display screen signal) may vary with light location due to spatial variations in

such quantities as local lifetime, it will not be influenced by the proximity

to a defect such as a region of poor metallization contact. In short, without significant internal shunting, the scanner is relatively insensitive to

the detection of such cell defects such as cracks, regions of poor contacting, regions of excessive emitter sheet resistance, etc., which may seriously

influence cell reliability and conversion efficiency.

The values of the shunt resistances can be varied by varying the diode forward bias. For the simple case in which all elemental diodes are equivalent

and all are biased to the same voltage, the shunt conductance per unit area is the same for all portions of the cell. For Cp representing this conductance, Pq representing the emitter sheet resistance, w and £ representing the

metallization stripe length and separation, respectively, the voltage across

the load Zj^ , i.e., the output voltage Vq^^ for the scanning light line

at the arbitrary position 0 _< p _< £ , is shown in section 4.6.2 to be _ (sinh[B(£ - p)] + sinh(Bp)}

out o £ (jsinh(B£) + [cosh(B£) - 1]

/ Po

where

3 = • < 2 )

As the laser light line is swept from one grid stripe to cinother, Vq^^^

undergoes a maximum-minimum-maximum excursion where the ratio of minimum to

maximum values is given by the following expression;

V ^ (min) out

V . ( max

)

out

sech( 3£/2

)

(3)

Equation (3) is plotted in figure 5 for a representative value of (3V2).

The desired forward biasing of the cell may be achieved by shining light on

the cell. It is appropriate now to relate the shunt conductance Op to the

cell current and associated air-mass-one (AMI) irradiance level. Figure 6

10
y

Figure 3. A cross section, not to scale, of an idealized solar cell con- nected to a load Z^, showing the portion between a contiguous pair of parallel metallization stripes.

Figure 4. The one-dimensional electrical representation of figiore 3 for the case of laser scanning with a line of light. The ceil emitter is represented

by the continuous resistive element, and the small-signal voltage-current ra- tios for the elemental diodes making up the cell between the stripes are represented by the discrete resistors shown connected with dotted lines eind the

line of light by an Elemental current source.

0—X—^ i

Figure 5. The variation of cell output voltage with scanning light position

between parallel metallization stripes for a representative value of (3£/2)

of 1.30.

11
DARK

a.

Figure 6. Idealized solar cell: (a) The I-V characteristics of the cell

under both dark and illuminated conditions. (b) The equivalent circuit where

the strength of the current generator is proportional to the illuminating

light intensity.

12
shows the I-V characteristics for the dark and illuminated idealized solar

cell and its equivalent circuit representation. The well-known effect of

shining light on the cell is to displace the I-V characteristic along the

I-axis an amount equal to the external current which would exist if the diode

were connected to a short circuit. This current will be designated 1^. An

alternate description states that charge flows through the cell in the

forward-bias direction. For the idealized case discussed with uniform illumination, 1^ is equal to the product of the cell junction area and the cell

current density <3ue to illumination. In turn, is related to the

cell voltage V and Jq, the junction saturation-current density, by

J_ = J - 1) . (4) L o In the scanning example described, the dc load to the cell is very large to

approximate an open circuit condition and is illuminated by a constant light

source while it is scanned with a much weaker (laser) source. The constant

light source causes the cell to be forward biased and produces a small-signal

junction conductance per unit area readily calculated by differentiating eq

(4) a = dJ^/dV = (qAT)(J + J ) . (5)

p L L o The current Jj^ will usually be several orders of magnitude larger than

Jq, and so the quantity multiplying (qAT) for all practical purposes

is simply Jj^. At room temperature, and with e^qiressed in amperes per

unit area,

a = 38.4 J, siemens per unit area . (6)

P L

The irradiance level for a representative cell and measurement condition can

be readily calculated. Assume a stripe separation Ji of 2 mm, an emitter

sheet resistance Pq of 50 '^/u, and 0.9 as the value of VQQ^(rain)/vQ^^(max)

judged to just yield adequate scanning sensitivity. From eqs (2) and

(3), the Op-value required for the assumed values of £ and Pq is 0.442

S/cm^, and from eq (6), the required value of light-generated current

is 11.5 mA/cm^. The short-circuit photocurrent from a good quality

silicon cell e}q>osed to an an irradiance of one sun with an air-raass-one

spectral distribution is about 38 mA/cm^ [3]

The required cell irradiance level is approximately 0.30 suns, a value which can be reached

quite easily in the laboratory. In figure 7, required irradiance levels are plotted versus desired scanning-sensitivity ratios for various com- binations of stripe spacing and emitter sheet resistance. The irradiance values required to give the unambiguous v(min)/v(max) value of 0.5

for line-scanning typical nonconcentrator cell designs having Jl-values

from 1 to 3 mm and p^-values from 50 to 200 ^/a can be achieved with an

irradiance source of 10 suns or less, according to this figure. Such an irradiance source has been constructed and is described in section 4.2.

The second way suggested to achieve internal resistive shiinting, i.e.,

by using a current source, is conceptually simpler than flooding the

cell with light. When an external current source is used, the cell

emitter-base junction is no longer uniformily biased as when a uniform light source is employed, and no steady-state current exists in the cell load impedance. The equations presented in this section hold only for zero dc load13
INSOLATION,

AMI

SUNS

Figure 7. Air-mass-one insolation levels versus desired scanning- sensitivity

ratios for various combinations of cell stripe spacing (£) and emitter sheet

resistance (Pq) for a junction silicon solar cell.

14
current conditions, and an analysis of the case for an externally supplied

electrical bias current is more complex. However, the cell scanning results

discussed in the following section show that the method of using an external

current source is a convenient way to detect a variety of cell defects.

4.1.2 Cell Scanning Results

4. 1.2.1 Emitter Metallization Breaks

Results of scanning a space-flight quality solar cell are shown in figure 8

(with a sketch of the cell) and are used to demonstrate the detection of

breaks in emitter metallization stripes. The cell was scanned while it was forward biased, either by light or by an external current source, or both.

Metallization breaks as detected with an optical microscope are shown by

dots in the sketch. Because of the electrical redundancy in connecting the

fine (50-ym wide) herringbone metallization grid lines in the central portion

of the cell to the wide (280-ym) metallization splines, two breaks in a fine

line are required to disconnect any appreciable length of fine-line metallization from the cell output electrode. The lower three photographs of figure

8 show the results of scanning with 0.633-ym laser light. No metallization

defects are observed with the cell unbiased (fig. 8c). With the cell forward

biased (figs. 8d and 8e), metallization defects are readily observed by the

lowered photoresponse in the vicinity of the unconnected metallization. The

metallization which is continuous to the splines appears to make uniform

electrical contact to the underlying emitter, as evidenced by the photoresponse attaining a maximum adjacent to the metallization. In figures 8c and

8d, the horizontal scan lines are deflected upward by an amount proportional

to the cell response to the scanning light spot. In figure 8e, the cell resx>onse is mapped in a more conventional manner, by modulating the display

screen intensity. The photoresponse presentation is such that maximum screen

darkness corresponds to maximum photoresponse. The resulting pattern outlines the metallization regions making effective contact between the cell

emitter and output electrode. For both figures 8d and 8e, 360 mA of the cell

current is provided by light and 126 mA by an electrical supply. Qualitatively, similar results were obtained with a more modest total cell current

of 126 mA supplied by the electrical supply alone (this 126-mA value corresponds to about 80 percent of the cell short-circuit current were it exposed

to AMI insolation). The larger 486-mA bias current value was chosen to en- hance the photographic presentation of the results shown in figures 8d and

8e. Even though scanning was performed with a spot of light, the general

shape of the photoresponse versus position away from the broken metallization

region is in qualitative agreement with the results predicted from the line- scan analysis performed in section 4.6.2 and with the generalized analysis in

section 4.6.3.

4. 1.2. 2 Metallization Not in Ohmic Contact with the Emitter Region

The ability of the scanner to detect portions of the metallization not in

ohmic contact with the emitter is demonstrated by the results of scanning an experimental polycrystalline silicon solar cell under development. The cell

is about 3.3 by 3.3 cm (1.3 by 1.3- in.). It is encapsulated in a soft,

transparent plastic which is about 0.25 cm (0.10 in.) thick at the surface.

15
e

.

Figure 8. A 2- by 2-cm (0.8- by 0.8-in.) silicon "space" ceil; (a)

Eliotograph of the cell. (b) Sketch of cell with metallization breaks shown

by dots. (c) Scanning photoresponse using the y-axis display mode and with

the ceil unbiased. (d) Scanning photoresponse using the y-axis display mode

and with a ceil bias of 486 mA. (e) Scanning photoresponse using the

intensity-modulation display mode and with a cell bias of 486 mA.

16
A photograph of the cell is i^own in figure 9a. The remaining photographs in

figure 9 are of the display screen where the cell was scanned with 0.633-ym

light. Figures 9b euid 9c are for the cell with no bias applied and 9d and 9e

are for the cell with a forward bias# supplied by an external current source

of 100 mA. The intensity-modulation display mode is used in figures 9b and

9d, and the y-deflection display mode is used in figures 9c and 9e. It is

sometimes desirable, as was done here, to use both these display modes separately. While the y-deflection mode is useful for quantifying cell defects,

some portions of the cell response may block from view the response from

other portions. By using the intensity-modulation mode first, one can view

the re^onse from all portions of the cell. The cell may then be repositioned, if necessary, for quantitative measurements of the photoresponse in

the y-deflection presentation mode. One must be careful vrtien comparing

intensity-modulation mode photographs, however. Unless the photographs are

made under identical electrical eind photographic conditions, e.g., the same maximum signal excursions, the same "white level" signal values, the same film exposure times, etc., comparison of the photographs may be misleading.

The results shown in figure 9 indicate that although the top metallization is

continuous to the output terminal, portions of this metallization are not

making ohmic contact to the underlying semiconductor. This is evidenced by a lowered photoresponse on both sides of the metallization areas in question

with the cell forward biased. This type of cell defect cannot be detected by

previously available techniques such as optical inspection or laser scanning

without bias.

In order for photoresponse at the edge of a solar cell metallization stripe

to achieve a coimnon, maximum value, the metallization must be continuous to

the output lead cuid must make a uniform ohmic contact to the underlying semiconductor. These are precisely the fundamental requirements for effective

metallization, emd so the scanning technique described provides a test for

cell metallization efficacy.

4. 1.2.3 Cell Crack Detection

The biased-cell scanning technique was also used to locate cracks in solar

cells. This is an important application because cracks have been known to

cause cell eurrays to fail, sometimes dramatically, with electric arcs and

flames produced at the crack [4] . In order for a crack to be observed visually, it must be wide enough to scatter the observation light. For cells

having a smooth surface, the width of the crack must be a fraction of a micrometer or wider. Solar cell cracks smaller than this, while not detectable, may widen in service. Cracks in cells with textured surfaces may be

even harder to detect visually because the light reflected from the surface

irregularities tends to obscure light scattered frcan the crack. These problems point to the need for a crack-detection technique which does not depend

on visual inspection. Crack detection with the laser scanner makes use of

the obvious consequence of cracking a semiconductor ^ecimen; the electrical

conductivity of a specimen becomes discontinuous at the crack. If the crack

rxins through the emitter of a solar cell, a discontinuity appears in the

three-dimensional resistive array which represents the small-signal equivalent circuit of the cell in forward bias. This resistive discontinuity can

be revealed by laser-scanning a forward-biased cell in the vicinity of the

17
a.

d . e

.

Figure 9. A 3.3- by 3.3-cm (1.3- by 1.3-in.) polycrystalline silicon solar

cell: (a) Photograph of the cell. (b) Scanning photoresponse using the

intensity-modulation display mode and with the ceil unbiased. (c) Scanning

photoresponse using the y-axis display mode and with the cell unbiased. (d)

Scanning photoresponse using the intensity-modulation display and with a cell

bias of 100 mA. (e) Scanning photoresponse using the y-axis display mode and

with a cell bias of 100 mA.

18
crack. One would expect resistive discontinuities to occur when the crack

width is equal to or greater than the distance for majority-carrier tunneling/ i.e./ for cracks on the order of 2.5 to 10 nm wide or greater. ThuS/

scanning under proper conditions can reveal cracks 10 to 200 times narrower

than can be observed under the most favorable conditions with visual inspection methods. It is not necessary for the scanning light spot diameter to be

the same order of, or smaller than, the crack width. It is only necessary

that the spot diameter be much less them the signal attenuation length, i.e.,

the distance over which the cell scanning response falls to e”^ of its

maximum value. The attenuation length may be varied by adjusting the cell

forward bias and is chosen typically to be one-half to one times the grid

separation distance [5].

With the cell forward biased, one sees a change in the cell's photoresponse,

the crack "signature," as one scans across the crack. Experimentally, one sometimes sees this manifested as a slope change or an amplitude change in

the photoresponse scan. A slope change could be caused by carrier recombination at the crack or by shunting leakage currents [5]. Amplitude discontinuities can be expected if the crack is closer to one metallization finger than

another [5]. A crack between a pair of metallization grids should not influence the photoresponse outside that region. Occasionally it does, and some areas of the cells delineated by cracks may exhibit a lowered scanning re- sponse everywhere. This could be due to differences in resistances between

the cell output electrodes and the various semiconductor regions contacted.

A number of cells were inspected for cracks. All cracks which were previously located with the use of a microscope were readily found with the scanner.

The cells were forward biased with values of current not markedly different

from those that would exist within the cells with the cells exposed to their

normal solar insolation. In some cells with smooth surfaces, for which visual infection should work best, cracks were visually observed to begin at

cell edges and appeared to end within a short distance. Laser scanning with

the cells forward biased easily confirmed the location of all cracks so located visually, and equally important, the crack signatures indicated that

the crack lengths were always greater than those visually observed. For some

of these cases, crack signatures were observed across the entire cell. Other

smooth- surface cells that had passed visual inspection as "crack free" were

similarly scanned, and some of these also exhibited crack signatures. One

practical implication of these results is that one may now determine vrtien a cell has developed a crack wide enough to influence its operation. In the

past, it has been difficult to say whether this occurred in array service,

whether this occurred during array fabrication, cr whether the cells for

these arrays were received in this state from the manufacturer. This que - tion can now be answered and should permit one to achieve enhanced array re- liability.

Figure 10 shows scanning results obtained from a 100-mm (4-in.) diameter

n-on-p single crystal silicon cell that was purchased commercially and found

to have a crack. The results of scanning two small portions of the cell illustrate both the slope-change and amplitude-change predictions discussed

above. The vertical lines are ^adows of the metallization stripes which are spaced 2 mm (0.08 in.), center to center. The diagonal "troughs" in the lower right-hand corner of figure 10a and 10b are shadows of the wider metalli19
c . d

Figure 10. The results of scanning two small portions of a cracked 100-mm

(4-in.) diameter silicon cell using the y-axis display mode. (a) Scanning

photoresponse of the first portion with the cell unbiased. (b) Scanning photoresponse of the same portion as shown in (a) but with a cell bias of 0.9 A.

(c) Scanning photoresponse of the second portion with the cell unbiased. (d)

Scanning response of the same portion as shown in (c) but with a cell bias of

0.9 A. The cell crack location is indicated by arrows in (b) and (d).

20
I.

zation buses which connect the metallization stripes to each other and to the

top output electrode.

'll

I Figures 10a and 10c show the scanning responses of the two cell portions when

I a zero-bias current is applied, while figures 10b and lOd show the responses

when a current of 0.9 A is supplied by an external current source. This cur- rent is about 29 percent of the current which the cell could produce were it

crack free and exposed to one sun insolation. For this cell, it was possible

I to determine the crack location by optical Inspection. The crack coincided

I with the scan response discontinuities appearing in figures 10b and lOd. In

j the display photographs, several surface features can be seen that are not

cracks but show similar zero-bias scanning signatures. These features appear

to be benign and may be due to scratches, dislocation lines, or the presence

of unwanted material. One such feature, which is at an angle to the grids in

figures 10c and lOd, was identified (with an optical microscope) as a thin

I

film of the gridding metal. Since cell cracks (when they are wide enough to

produce photoresponse perturbations without the cell being biased) and cell

blemishes can yield the same zero-bias scanning signature, the harmless features could be mistakenly labeled as cracks and the cell rejected needlessly.

Alternatively, a true crack might be mistaken as a benign cosmetic blemish.

This set of photographs suggests that separation of these classes can be accomplished with forward-bias scanning.

1 4. 1.2. 4 Other Scanning Results

The following solar cells and cell prototypes have been laser scanned using

the techniques described: GaAs concentrator cells, Cu2 S/CdS and

Cu2 S/Cd[Zn]S cells, a thin-film polycrystalline silicon cell, EFG polycrystalline silicon cells, MIS single-crystal GaAs prototypes, silicon vertical

junction and tandem junction cells, and both n-on-p and p-on-n junction flatplate silicon cells. In addition to detecting metallization defects and

crystal cracks described in the previous sections, the technique described

has been used to:

I 1. Identify some portion of an MIS cell with significantly smaller photoresponse them the rest of the cell. This is felt to be due to "island formation," e.g., break-up of the thin, optically transparent metal film which

may occur in these structures during storage or during service.

1

!| 2. Provide information useful to the cell design physicist. For exam1 pie, it was found for a manufacturer's run of polycrystalline silicon cells

I that the same cell areas which had a zero-bias 1.15-ym photoresponse about

I one-half of that for the rest also showed a decrease in the 0.633-ym photoj response for forward-bias currents corresponding to irradiance values of

about one sun. The first observation suggested that the areas in question

had poor back contact reflectance, and the second indicated poor ohmic con- tacts. Combining these two observations permitted the underlying cell design

and fabrication problem to be understood and corrected.

I

I

I 4. 1.2. 5 Further Applications of the Laser Scanner

I

I Experience gained during the course of the observations reported above leads

to the following suggestions for utilizing the solar cell scanning method:

-21
1. Cell metallization-efficacy testing can be done under production

line conditions by unskilled personnel. With the equipment set up for a particulau: cell type and using the intensity-modulation display mode, one only

requires that the operator note if a "deurk line" (shadow of the metallization

stripe) rvins through the center of a "deurk region." No knowledge of devices

or fabrication processes is required of the operator.

2. By dividing the scanning response under bias with the zero-bias re- sponse on a spatieLL point-by-point basis, the fabrication and service-related

cell loss mechanisms should be even easier to perceive and measure. The eye cem make comparisons of this sort to some extent, but it might be better to use a computer.

3. The scanning technique developed may be very useful for monitoring

solar cell degradation processes. For exeunple, it is quite possible that

early changes in metallization efficacy for GaAs concentrator solar cells

could be detected 1:^ the technique. This cell type, because of its severe service operating conditions of large diurnal variations in temperature,

large cell current densities, and possible exposure to corrosive environments, might be expected to i^ow significant changes over its lifetime.

4.2 Equifxnent for Light-Biasing Cells

The analytical results presented in figure 7 show that in order to reliably

observe cell defects such as cracks and metallization discontinuities in

flat-plate (nonconcentrator) cells, the illuminator for forward biasing the

cells during scanning should have the capability of producing cell shortcircuit current response equivalent to exposing the cell to an insolation of

about 20 suns. Noise and modulation of the light by, e.g., an inadequately

filtered lanp power supply, should be minimized; otherwise, the scanner

signal-cUBplifiers will process this extraneous optical signal cLLong with the

cell response to the laser scanning spot and obscure the display information.

Various light sources were investigated. Those considered first were highpressure xenon gas discharge lamps such as are used in solar simulators. One

such l2unp was set in a test bench and powered by a filtered dc source. Monitoring the light output with a photodiode revealed spikes and noise

bursts which at times exceeded ten percent of the average light output and

which had frequency components measurable to about 35 MHz. A dual-trace oscilloscope simultaneously monitoring the light output emd the lamp voltage

showed that the extraneous light signed, originated within the lamp; undoubtedly, this is associated with instabilities inherent in the plasma discharge.

It was concluded that, while such sources may be "quiet" enough for use in

solar simulators where one is concerned with the average value of the light,

they are not suitable for use as a laser scanner bias source.

Quartz-halogen incandescent sources were next investigated. A new model

quartz-halogen high-intensity illuminator designed to be used with the same model microscope as the one incorporated in the scanner was tried, but the

results were disappointing; the short-circuit cell current for a small area

(2- by 2-cm) (0.8- by 0.8-in.) cell was less than one-sun irradiance. An ex- amination of the optical path between the lan^ and the specimen stage showed

22
that although the illuminator was more than adequate for its intended usage,

it was not designed for efficient utilization of the lamp's output, and the

optics between the illuminator and the i^ecimen constituted a high f-mamber

system. It became apparent that an illuminator tailored to the project needs

had to be designed.

An insolation source was successfully designed around a quartz-halogen highintensity lan^) by using four flexible fiber-optic light pipes to direct the

light to the cell area being scanned. With this new source, irradiance levels of about 20 suns can be produced at the surface of cells measuring 1 cm by 1 cm. The optical system uses four groups of heat filters and condensing

and focusing lenses arranged symmetrically around the long axis of the lamp.

This arrangement captures most of the light available from the lamp.

An elusive source of modulation of light from the insolation source was located and eliminated. Vibration from the lamp cooling fan fastened to the

bottom of the leunp amd lens assembly housing was found to be the source of

the undesired modulation. Any motion of the light-source components relative

to one another can change the amount of light transmitted to the light pipe

bundles and, hence, can change the light emerging from the bundles and incident on the cell specimen. The problem was solved by providing a separate

support for the launp housing euid the cooling fan. This possible source of

vibration should be kept in mind by anyone attempting to duplicate the lightbias equipment.

A photograph of the light source with the top cover removed is shown in fig- ure 11. A small silicon solar cell, mounted inside the illuminator, powers

an external meter to monitor the light from the lamp. This circuit permits

one to set the lan^) power easily. Because neither the uniformity nor the

exact value of the cell illumination was found to be a critical parameter

when the illuminator was used with the scanner to detect cell defects, this

technique provides adequate control of the light output at the ends of the

fiber-optic pipes. Figvire 12 gives an overall view of the illuminator with

fiber-optic light pipes. Figure 13 is a photograph of the scanner taken

shortly after the illuminator was put in service. Figure 14 is a close-up

view of the ^ecimen stage; the cell being scanned is a 2- by 2-cm "^ace"

cell. The clan^) arrangement visible in figures 13 and 14 was an interim

arrangement to allow the light-emitting ends of the fiber-optic pipes to be

positioned relative to the cell. A more versatile arrangement is described

in the following paragraph.

For a given cell, one may need to use a variety of scanner lenses to investigate various ceil phenomena. The cell height relative to the scanner frame

may change as one changes from one lens system to another. The position at

the fiber-optic bundles must be adjustable to accoiraaodate these changes in

cell height and also to allow one to modify at will the pattern of bias light

at the cell, as one changes from one cell to another having a different area.

To satisfy these needs, a very versatile clamping system was constructed

which permits each set of two fiber-optic bundles (one set on the left and

one on the right) to be moved vertically and horizontally. Additionally,

each of the four bundles can be individually oriented. This system can be

seen in figure 15, although the light-biasing apparatus was not in use vdien

the photograph was taken. This picture was made near the end of the report23
24
Figure 12. An overall view of the high-intensity light source including

fiber-optic light pipes.

I

il

25
s<

p

Figure 13. The illuminator being used during the laser scanning of a solar

cell.

26
I Figure 14. A close-up view showing the means used to adjust and clamp the

I light-emerging ends of the fiber-optic light pipes.

I

!

I 27
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28
ing period, and it shows the microscope replaced by other optical elements;

it also shows ancillary apparatus used and developed during the period. The

other optical arrangements and the ancillary apparatus are discussed in sec- tions 4.3 and 4.4, respectively.

4.3 Laser Flying-Spot Scanner Modifications

A series of modifications was made to the scanner so that the cell size that

can be scanned at one setting of the equipment was increased. Before the

solar cell program was begun, the scan excursion was limited to a square area

about 3 mm on a side by the optical parts available for the particular microscope used for the scanner optics. This excursion was adequate for scanning

integrated circuits, discrete diodes, and transistors to obtain useful information about the point-by-point spatial operation of these devices (Appendix

A), but most solar cells are larger than 3 mm square and so could not be

scanned without repositioning. By changing the objective lens system for the

original microscope, the scan excursion was increased to 11.5 mm, and the

working (objective lens to specimen) distance was also increased to permit

the inclusion of the optical-bias fiber-optic pipes described in section 4.2.

Additional increases in scan area and working distance were achieved by re- placing the entire microscope optical system (with the exception of the microscope ocular lens) with a 35-mm single-lens-reflex camera lens having a focal length of 55 mm. The maximum scan excursion is now 100 mm (4 in.) with

a spot size of about 25 ym. The resolution achieved with this spot size is

adequate to resolve the finest metallization grids found on nonconcentrator

solar cells.

Figure 15 shows the photoresponse of one of the 5.63-cm (2.25-in.) diameter

cells in a 36-cell module scanned with 0.633-ym laser light. The picture was

made with a time exposure of about 5 s to capture the entire display screen

image of the cell photoresponse. A flash exposure was used to superimpose an

image of the equipment on the film negative. The frame fixture for the 25-

by 60-cm (10- by 24-in.) cell module was constructed with a clan^) arrangement

which can be slid along the long dimension of the module and locked at any

location desired. Electrical contacts are carried on spring-loaded probes

traveling on parallel rods at right angles to the long dimension of the frame

and module. Connections to the desired cell are made by pressing the probes

throuc^ the transparent plastic array cover to contact the cell output

leads.

4.4 Ancillary Apparatus

4.4.1 Solar Cell Impedance-Matching Network and Preamplifier

The capacitance associated with the injection of minority carriers in

forward-biased solar cells may severely limit the spot deflection scanning

rates allowable, when one employs the method to pin-point cell defects described above. One way of solving this problem is to reduce the RC timeconstant of the cell and its load. This reduction can be accomplished by

coupling the cell to the scanner display electronics with a transformer input. By matching the low dynamic resistance of the cell to the display input

resistance with a suitable transformer, one also maintains the cell's intrin29
sic signal-to-noise ratio, v^ich would otherwise be degraded if one simply

connected the cell to the hi^ input impedance of the display electronics.

Because the cell is forward biased during scanning, one cannot connect the

transformer directly across the cell, but a dc blocking capacitor must be

placed in series with the transformer primary. The reactance of the capacitor must be small compared with the transformer input impedance at the lowest

frequency component originating from the scanned cell.

Figure 16 is the schematic diagram of the complete coupling network. Two

56,000 yF, 30-V dc electrolytic capacitors, labeled , C2

, are connected

back-to-back yielding an effective value of 28,000 yF. This arrangement allows cell biases (relative to the cell "base") to be either positive (for

p-on-n cells) or negative (for n-on-p cells) without danger of capacitor damage. As a rule, the impedance of electrolytic capacitors tends to rise with

frequency. The purpose of C3 is to provide a low impedance path at these

hi^er frequencies. An 80-V dc low-loss capacitor having a value of 0.5 yF

was found to be a suitable choice for C3. The wire-wound noninductive re- sistor connecting the cell to the bias supply has a resistive of about 15 Tii. The impedance matching resistor across the transformer secondary has a resistance of 50 kJ^. A stable, adjustable, and well-filtered dc source serves as the bias supply. Other general-pvirpose laboratory power supplies have also

been tried and found to be satisfactory.

A number of matching transformers, T-|, each having input impedances of the

order of 1 were tested. One test was to measure the frequency response of

the transformer under impedance-matched conditions to simulate actual operation. To achieve impedance matching, a simple resistive network was inserted

between the signal source and transformer to present a 1 -fi effective source impedance to the transformer primary. A 50-kJ^ impedance matching resistor

was connected across the transformer secondary. The criterion for this test

which used to be transformer selection option was the separation of upper-tolower cutoff frequencies. The larger the separation, the better the transformer. The other test, also performed under impedance-matched conditions, was of the response of the transformer to a simulated video signal. The re- sponse was tested by using square waves with a repetition frequency in the

middle of the transformer's pass-band. The criterion for selection was that

the transformer output waveform should be a good replica of the input waveform, that is, no significant overshoot, droop, or ringing should be observed. The transformer chosen as the most suitable for Incorporation into

the circuit had been designed to serve as a step-up transformer for coupling

a phonograph cartridge to a preamplifier in a high-fidelty audio system.

Compared with simply connecting a solar cell to the scanner display electronics, use of the transformer increased the signal about 35 dB and increased

the signal-to-noise ratio 30 dB. The completed matching network, which has a flat frequency response from 5 Hz to 13 kHz (3 dB points), is housed in a 5.1- by 7.6- by 12.7-cm (2- by 3- by 5-ln. ) aluminum rectangular box.

The preamplifier is the plug-in signal amplifier of a general purpose oscilloscope, and it is powered by the oscilloscope. The amplifier has a front

panel output jack which feeds the scanner display. The solar cell scanning

response is also displayed on the oscilloscope screen for direct monitoring.

30
DC

Figvire 16. Schematic diagram of the solar cell

coupling network. The criteria for the selection of

the components emd the component part values are stated in the text.

31
The plug-in amplifier has gain controls which permit one to maintain, by mon- itoring the oscilloscope presentation, the peak-to-peaJc signal level fed to

the display screen electronics for bias-on, bias-off cell scanning measurements. This allows one, by glancing from one photograph of the display

screen to another, to directly compare the effects of cell biasing. This

system was used to produce most of the photograph sets shown in section

4.1.2. The noise characteristics of the plug-in amplifier were measured, and

it was shown to produce only a few decibels of noise above the thermal level

for frequencies within its pass-band when its input is connected to a 50-kfi source impedance. The amplifier has separately adjustable low and high cutoff frequencies. By judicious selection of these frequencies, specimen noise

outside of the cell scanning response spectrum is reduced to enhance the

clarity of the display screen presentation.

4.4.2 Solar Cell Clamping and Rocking Stage

Most ICs and discrete transistors, studied before the present reporting period, were easy to mount for scanning since they usually ccune equipped with

pins or wire leads which could be inserted in appropriate sockets clamped

onto the microscope specimen stage. The plane of the scanned device could be

easily adjusted by bending the device leads so that the light reflected from

surface features such as metallization is directed toward the germanium photodiode used in the reflected-light circuit. However, neither the same holding arrangement nor the same method for aligning the reflected-light path is

workable with the usual solar cell. The cell is usually flat, and if leads

are an integral part of the cell, they are not normally adequate to hold the

cell. To solve this problem, the clamping and rocking unit shown in figure

17 was designed and constructed to fit between the solar cell and the

microscope x-y cross- slide stage; the x-y stage retains its original function

of translating the scanned specimen perpendicular to the optical axis of the

microscope. The upper part of the unit is a vacuum chuck having replaceable

top plates to accommodate cells of various sizes. The rocking feature is

controlled by the two micrometer heads that can be seen in the photograph.

These permit precise rotation of the vacuum chuck around the x- and y-axes to

direct the reflected light to the gernumium £^otodiode. The chuck is i^ring

loaded against the micrometer tips to maintain the settings of the micrometer

heads.

4.5 Reference Structure Arrays

The results presented in section 4.1.2 show that the scanning techniques developed are quite effective in revealing cell defects. These results are qualitative, and one also needs to know the precision of the techniques developed. With this known, one would able to measure defect magnitudes and

cell parameters with a stated measurement accuracy to put the laser scanning

technique on a firm quantitative basis.

To accomplish this. Reference Structure Arrays (RSAs), which suce solar cells

incorporating precisely known "defects" and known pareuneter variations and

which are accompanied by eincillary test patterns, are to be fabricated and

laser-scanned under planned bias and scanning conditions. The test patterns

are designed to give independent measurements of the "defect" euid parameter

values. The laser scanning results are then to be compared with results ob32
Figure 17. Solar cell clamping and rocking unit attached to the microscope

cross-slide stage. Cell clamping is performed with a vacuum hold-down chuck

magnetically held to the top plate of the rocking mechanism. Rocking (the

orientation of the vertical axis) is controlled by micrometers. This photograph was made with the scanner in operation, and the scanning light raster

is approximately in the middle of the cell.

33
tained by mathematical modeling such as described in section 4.6.3. During

this reporting period, the design of the RSA was completed. The RSA consists

of four solar cells, each surrounded by test patterns to give a mean value

of, and the spatial variation in, cell and material properties and parameters

including emitter emd metallization sheet resistance, metal-to-emitter con- tact resistance, substrate resistivity, and carrier density in the base as a function of distance below the emitter-base junction. The first solar cell

is "perfect," i.e., it contains no intentional defects; the second contains

metallization-emitter open regions of various controlled sizes; the third has an emitter sheet resistance which varies periodically in a stripe pattern;

and the fourth contains both opens and the periodically varying emitter sheet

resistance. The RSAs will be constructed in single-crystal silicon, as only

silicon technology offers the fabrication precision required.

4.6 Mathematical Analysis

4.6.1 Laser Scanning Using Microwave-Modulated Light

It is well known that the energy conversion efficiency of a cell may be se- verely reduced if the lateral resistance of the emitter is excessive, since

all the photocurrent is constrained to flow laterally along the emitter.

Thus, almost all junction solar cells employ gridding to reduce this resistance. Several gridding schemes may be employed. For exeunple, the grids may

be in the form of radial "^okes," or they may be in a cross-hatch pattern,

but the most common pattern is a "finger" configuration. This is the configuration which is assumed in the analysis which follows. The purpose of this

analysis is to determine the conditions under which the laser flying-spot

scanner, using light modulated at high frequencies, can be used to determine

in a nondcunaging manner the sheet resistance of the emitter. With this known

through measurements made on the cells themselves, one can attempt to control

its value on later cells and optimize cell performance. After techniqueshave been developed for 'in S'itu emitter ^eet resistance measurements, one

can then work toward developing techniques for measuring variations in this

resistance. The emitter sheet-resistance measurement technique which will be

described uses only the contacts to the cell (the electrode terminals) which

are always an integral part of the completed cell.

Figure 18 is the representation of a reverse-biased solar cell with grids in

the finger configuration showing the passive cell elements. (If the cell were not reverse biased, the equivalent circuit would include additional passive elements representing the junction diffusion admittance.) The state of

affairs depicted corresponds to an instant in time when the laser spot is approximately midway between the two grid stripes, shown by shading in the figure. The device equivalent circuit is that of a distributed resistancecapacitance network due to the emitter sheet resistance and the transitionregion capacitance of the reverse-biased p-n junction. The net result of

carrier photogeneration due to the laser spot is to place a current generator across the transition-region capacitance jus;t below the light spot [6,7,8], as shown in the figure. If the light is modulated, then the equivalent cur- rent generator will have both ac and dc components. (There will always be a dc component, because it is not possible for light to have an average value

of zero!) Intuitively, one would expect the ac voltage measured between the

34
I Figure 18. Representation of the two-dimensional parasitic resistancej capacitance nature of a reverse-biased solar cell.

!

I

:i

; I

!

^ 35
top and bottom electrodes to vary with the location of the light i^ot, reaching a maximum just at the edge of a grid and a cononon trough-shaped minimum

midway between a pair of grids. One woxild expect this because of the symmetry of the grid pattern and because of greater attenuation of the signed, current by the two-dimensional resistance-capacitance network as the distance

between the light spot and its nearest grid line is increased.

For the geometry shown, the analysis is simplified if a line of laser light

parallel to the grids is used, rather than a spot, and the scanning is performed by moving the light line in a direction perpendicular to its length.

The grids and this line of light are assumed to have the same length w. If

the line is much longer than the separation of the grids, the problem can be

treated as one-dimensional. Figure 4 is the equivalent electrical circuit of

the portion of the solar cell between two grids separated, edge to edge, by a distance £. Although scanning with a line of light will probably not be performed in the program, it is useful to analyze this one-dimensional case be- cause the solution can be obtained in a closed-form expression which may be

excunined to anticipate the important results for the more realistic twodimensional case. In figxire 4, the scanning line is at a distance p frcm the

edge of the left grid. It is clear that all such cell portions are eq[uivalent. It is only necessary to analyze one. The impedance of the rest of the

solar cell (as yet unspecified in value) may be lumped with the cell terminating impedance (also unspecified) to yield the load impedance Z^.

The equations relating the line current i(x) in the plane of the emitter, the

voltage v(x) across the line, the capacitance per unit area c of the line,

the emitter sheet resistance Pq , the angular modulating frequency w, eind the

coordinate distance x are easily derived and are

di(x)

dx

= - jwcwv(x) (7)

where j = (-1)^/^.

dv(x)

dx w

i(x) ( 8 )

Differentiating eq (8) and substituting into eq (7),

- e^(x) = 0 (9)

dx

where

6 = (jw/wg)^'^^ / ( 10 )

and (i)Q is the angular frequency

o)o = (PqC)

-1

( 11 )

The general solution of eq (9) is

, . , 3x ^ „ -3x v(x) = Ae + Be ( 12 )

36
The constants A and B are chosen to satisfy the boundary conditions. From eq

(8), the line current is

i(x) Be (13)

Two sets of equations similar to eqs (12) and (13) are required: one to represent the line to the left of p, and the other for the line to the right

of p. Designating the quantities to the left by the subscript 1 , and to the

right by the subscript II, and with iQ representing the magnitude of the

current generator due to the modulated laser light, the two sets of equations

are solved subject to the boundary conditions

ill(p) “ ij(p) = M4)

Vi(p) = Vjj(p) (15)

Vi(0) = Vjj(£) = Z^[i^^(5,) - ij(0)] . (16)

The voltage across the load Z„, i.e., the output voltage Vq^^ for the

scanning li^t line at the arbitrary position 0 £ p ^ is

. „ {sinh[3(& - p)] + sinh( 3p) }

^out

“ ^0^ f 2w3Z,

I*

jsinh(3Ji) + [cosh(3^^) “

As the laser light line is swept from one grid stripe to another,

undergoes a maximum-minimum-maximum excursion having the ratio

V ^(min) out

V ^(max) out

out/p - 1/2 ^ (3V2)

out/p = 0

(18)

It is noteworthy that this ratio does not depend on the effective load impedance Z^. All the quantities in eq (18), with the exception of Pq , are known,

or can be found readily for the cell using nondamaging methods, and so se- lecting a value of Pq which brings eq (18) into agreement with the measured

ratio VQ^^(min)/vQ^^(max) yields the emitter sheet resistance.

4.6.2 Laser Scanning of Forward-Biased Cells.

For forward-biased cells scanned with a line of light, the pair of equations

analogous to eqs (7) and (8) are

di(x)

dx

wo v(x)

P

(19)

dv( x)

dx ( 20 )

where is the shunt conductance, due to forward-biasing, per unit area,

and p^ is the emitter sheet resistance as before. By changing the definition

of 3 used in section 4.6.1 to

37
the resulting expressions for the output voltage, and the roaxiraumrininimummaxlmxim voltage excursion are again given by eqs (17) and (18).

4.6,3 Mathematical Modeling Work Performed Under Subcontract with the

University of Southern California*

4. 6 . 3.1 Introduction

D. Sawyer and coworkers at NBS are scanning solar cells by a modulated light

spot as a means to locate and identify cell faults by inspection of the cell

output pattern. Their aim is the modeling of various faults and the computation of the resulting output patterns. Comparison of the computed patterns

with observations will assist in identifying the faults in e}q>erlmental

The solar cell will be modeled by a two-dimensional distributed network of

diodes connected by semiconductor sheets having sheet resistances ptp on top

of the junction and P 3 on the bottom. The corresponding bulk resistivities

are flip and flg, respectively. An edge view of the model is shown in fig- ure 19, The effect of light is to place a current source of total magnitude

II across the diodes illuminated, A top view of the electrode configuration is shown in figure 20. The electrodes are of length W. Figure 21 shows

an element of the distributed network of figure 19 and indicates the notation

used in the analysis; (x - x-j) is a delta function centered at x = x-j

The notation used here is consistent with that used in reference [9]

4. 6 . 3. 2 Basic Equations

The dark current per unit area Jq depends exponentially on the voltage across the junction;

The pre-e;qponential coefficient Jq is the reverse saturation current per

unit area and B *= q/akT with a the diode quality factor.

Lateral diffusion currents are neglected so that the lateral sheet current

densities are

cells.

BV

1 ) (22)

(23a)

and

J (23b)

*This section of the report was prepared by K. Lehovec and A. Fedotowsky

Department of Electrical Engineering, University of Southern California

Los Angeles, CA 90007.
Figure 19. One-dimensional cross-section through the equivalent circuit of a

soleu: cell portion illuminated by a modulated light beam at x = • Output

is ac-shorted by a large capacitive load.

39
X —

Figure 20. Sketch of a solar cell i^owing finger

electrodes spaced by distance s and cm illuminated spot

at (x^» 0).

Pt/dx

Ps/<l«

Figure 21. Element of the distributed network shown in

figure 19 indicating the notation used in section 4.6.3.

Jodx(eBV-i)ii

ysAvJvwvj

40
where the subscripts refer to top and substrate (l.e«, base) of the junction,

respectively. Conservation of charge requires that

- n^/Pj

'^L '^D '

and

'"l -"'d = - '2'“”

where Qrp and Qg are the net charges per unit area above and below the

metallurgical junction. Charge neutrality in any vertical section of the

panel requires that 3Qij./3t = - 3Qg/3t 5 3Q/3t. Substituting eqs (23a)

and (23b) into eqs (24a) and (24b), respectively,

and

so that

V^V = V^(V^ - Vg) = - (p^ + pg) . [J^ "

Now 3Q/3t H 3Q/3V • 3V/3t = C3V/3t, so that

V^V = - p[J - - C3V/3t] (27)

Xj d

with p E p,j, + pg the sum of the sheet resistances and C = 3Q/3V the incremental diode capacitance per unit area.

4. 6. 3. 3 Low Intensity, Modulated Illumination

Next, consider the case of a generation current comprising dc and ac components :

= J. + . (28)

ll ll ij There will then be both dc and ac voltage components:

V = V + V (29)

'\j and eq (27) separates for small ac signals (BV << 1) into the relations

A = - ptj^ - jpi

and

2% % 'V, V V = - p[Jj^ - YV] ,

where

(30)

(31)

41
Y = G + jo)C

D

( 32 )

is the diode admittance per unit area and Gq = B(Jq + Jq) is the incremental diode conductance per unit area.

In the unilluminated region

2'\j 'X, V V = VYp . (33)

Since Z = Z(V), there will be in general a positional dependence of p/Z.

Equation (33) defines a position-dependent complex characteristic length.

For scanning in ^e x-direction with a line of light oriented parallel with

the electrodes, V will vary only along the x-direction and the characteristic

length may be designated Xp. It is convenient to work with the inverse of

Xp. The inverse has both a real and an Imaginary component, and these may

be designated (Xp')“^ and (Xp")"^, respectively,

X = (X •)"“' + j(x ")"”* = (Yp)^/^ . (34) o o o The real part of this length provides an attenuation of V(x), while the

imaginary part provides an oscillatory component. The characteristic decay

length is

x^ ' = /l/p| Y I /cos ( (|)/2 ) ( 35

)

and

Xp" - A/(\ Y I /sin(4>/2) . (36)

The wavelength of the oscillating component is

\ = 2irx " = 27t/1 /p| Y I /sin(({)/2) , (37) o where

<p = tg"^ (wCI^) (38)

and

Y I = + o)V)^^^ . (39)

4. 6. 3. 4 Fault-Free Cells Having Uniform Attenuation Lengths

These considerations are applied to the special case of a fault-free cell

operated under the conditions that Xq defined in eq (34) is independent of

position. This can be realized by using a cell without external bias and

without background illumination, and by modulating the scanning light beam at

a sufficiently high frequency that Xq has the desired value of about the

finger electrode spacing s.

While the solution for line illumination of the perfect cell has previously

been derived by D. Sawyer [10], its derivation here is still of interest as a

42
starting point for modeling of faulty cellS/ completed subsequent to the

present reporting period.

4. 6. 3. 5 Fault-Free Cells Illuminated by a Line-Shaped Light Beam

This line shape is of interest since it leads to simple analytical solutions.

In case of only one pair of electrodes (at x = 0) of length w, one electrode

contacting the top semiconductor sheet and the other contacting the bottom

semiconductor sheet, the small-signal photovoltage for a short-circuit load

is

V(x) »

-x/Xfl

e sinhCx/x^) for x £ x^

-x/Xq

s sinhCx^/Xp) for x x^

(40)

The photocurrent crossing unit length of a line parallel to the finger electrode and located at0^x£x-j is

a. J ejq>(- x^/Xq) cosh x/x^ (41)

The short-circuit output current is

^sc * \ ’'/’‘o'

• (42)

The quantities xq ' and Xq" can be obtained from the magnitude

I ^SC I “ I ^L I ^

and phase

'

’‘/’‘o"

of the short-circuit output current. The limit for Ig^^ for x-j -> 0 is

Il» From these data, Rp/p and CRq can be derived. Thus, knowing the

dc diode conductance per unit area, l/Rp, C and p can be determined.

The potential ^(x) in eq (40) has been composed of two ejq>onential terms to

satisfy the boundary conditions V(x) = 0 at the electrode x = 0. If a second

finger electrode is considered at x = s > x-j , the boundary conditions ^(x)

= 0 at X = 0 and x = s may be satisfied by adding an infinite series of potentials from line source images with respect to these electrodes. This is a well-known procedure in electrostatic and optical problems. The result is

V(x)

pxL^ 0

sinh(s/Xp)

sinh(x/xQ )sinh[ (s-x^ )/Xq] for 0 £ x £ x^

sinh[(s-x)/xQ] sinh(x^/xQ) for x^ £ ^ £ s

(45)

and

43
Igc =* ij^{ [sinh[(s-x^ )/Xq] + sinhCx^/x^)] /sinhCs/x^)} (46)

where the first terra in brackets results from the current collected by the

finger electrode at x = s, and the second terra results from the current collected by the finger electrode at x = 0.

4. 6 *3. 6 Fault-Free Cells Illuminated by a Point of Light

The illumination of the point x-j, 0 induces the current source Ijj at

x-j# 0 in the equivalent circuit of figure 19. The resulting potential in

the case of the presence of only one set of finger electrodes located at x =

0, i.e., assuming that s - x^ >> Xq, is

« (5:_p/2Tr)[K (r*(x,)) - k (r*(- x, ))] (47)

L'^ O 1 O 1. where

r*(x^) * [(X - x^)^ + y^l^'^VxQ (48)

and Kq is a modified Bessel function of the second kind. The second terra in the bracket of eq (47) results from the mirror image of the current source

with respect to the finger electrode. The short-circuit current collected by

the finger electrode is

K (i4 y^/x ) dy , (49)

/ 2 . 2

/x^ + y

assuming infinite finger electrode length. The Integral is a simple case of

the general Sonlne-Gegenbauer Type [11]

:

^so

Note that the current for the point illumination is Identical to the current,

eq (42), for the line illumination. This result illustrates the equivalence

theorem (to be proven generally in the next report) of current generated by

point and line illumination. This equivalence theorem also applies to the case of two finger electrodes so that eq (45) derived for line illumination

applies also to point illumination.

4. 6. 3. 7 Output of Faulty Cells Having Noncontacting Electrode Sections

The general solution to the potentied problem of a point current source located at r-| * = (x.» y^) surrounded by an atrbitrarily shaped ac grounded

electrode contour r is

(V(r) - G(r^, r) - JJ(r')G(| r - r'| dr' (51)

SC

= P ’^r av/9x)

I •'~eo jjpsQ

dy

>\j = (l^x^Axof

44
-r

I -rI -f v^ere G(r-j, r) = G(| r - r^i |) is the potential at r due to point source

located at r-j , and 3'(r') is the current density entering the electrodes.

This current density satisfies the integral equation

G(r , r') = /3'(r')G( p

- r'| )dr' (52)

*^r thus assuring that V(r') = 0. The effect of a poor electrode contact is

taken into account by excluding the noncontacting electrode section from the

contour. The poor-contact problem is mathematically related to a diffraction

optical aperture problem.

45
5. WORKSHOPS

5.1 Stabilility of (Thin Film) Solar Cells and Materials

A workshop on the Stability of (Thin Film) Solar Cells and Materials was con- ducted at the National Bureau of Standards on May 1 to 3, 1978, as part of

the Department of Energy's National Photovoltaic Program. The workshop prograun as issued to the attendees is included as Appendix B. The workshop addressed many of the problems and obstacles to achieving stability and long

life of terrestrial, solar cell devices using current and projected technology. The following three groups of exploratory solar cell materials and con- cepts were considered: (1) [CdZn] S/CU2

S, CdS/Cu-Ternaries, CdS/InP, and

amorphous Si, (2) polycrystalline, MIS, and conducting oxide Si, and (3)

polycrystalline and AMOS GaAs.

Researchers in the field reviewed modes and mechanisms for failure and degradation of these systems and the status of present reliability testing.

Speakers from related device technologies discussed measurement and test approaches that they have used to achieve high device reliability. Two talks

of particular Interest described a technique for detecting changes in the

physics of failure with increasing severity of a stress test to establish the

maximum possible acceleration factor for a given failure and a technique for

uncoupling different failure mechanisms from an analysis of failure rate data.

Discussions in the working groups of the workshop led to the identification

of a number of needs that require attention before meaningful headway can be

made in the achievement and quantification of stability and long life for

solar cells developed using forefront materials and device technologies.

Among the conclusions reached by the 102 scientists and engineers who attended the workshop was that more research needs to be conducted in this

advanced technology. They identified the need to study the chemical,

mechanical, and physical compatibility of the various materials used in these

solar cell devices ^md their influence on device characteristics. Also

expressed was the need for the development of performance and measurement

standards for use in characterizing these devices. Finally, the need for

adequate device manufacturing process controls and adequately described test

procedures and data was underscored as a prerequisite for meaningful headway

in the quantification of solar cell stability and long life. The proceedings

of the workshop have been issued as NBS Special Publication 400-58.

5.2 Photovoltaic Material and Device Measurements

The Electron Devices Division of NBS and DOE/SERI began plans for a workshop on photovoltaic material and device measurements to be held in the Washington, DC, area in the ^ring of 1979. The purpose of the workshop is to

accelerate the development of thin film solar cells by improving the versatility and reliability of material and device measurement techniques. Attendees at the DOE Annual Review Meeting for the Advanced Materials R&D

Branch, Vail, Colorado, October 24 to 26, 1978 were given a questionnaire

soliciting their thin film material and device measurement interests and perceived needs, and the workshop structure was conceived accordingly. The preliminary plans are included as Appendix C.

46
6. CONSULTATION AND LIAISON ACTIVITIES

A proposal for a novel photovoltaic converter was reviewed by D. E. Sawyer

for DOE Headquarters.

D. E. Sawyer assisted in evaluating replies to DOE's RFP in Photovoltaic

Mechanisms in Polycrystalline Thin Film Solar Cells, DOE Headquarters, July

24 to 27, 1978.

At the suggestion of the SERI Photovoltaic Program Office, D. E. Sawyer attended the December 18th Photovoltaic Session of the Workshop on Reliability

of Materials for Solar Energy at Denver, Colorado, and participated in the

session discussions. This workshop was organized by the Materials Branch of

SERI's Research Division.

Nine solar cell invention submissions were given first-level review by D. E.

Sawyer for the NBS Office of Energy-Related Inventions.

47
7. REFERENCES

Sawyer, D. E., A Technique for Using An Optical Scanner to Reveal Solar

Cell Defects, Proa* 13th IEEE Photovoltaic Speaialists Conf,, Washington, D.C., June 5-8, 1978, pp. 1249-1250 (1978).

Sawyer, D. E., Kessler, H. K. , and Schafft, H. A., Solar Cell Measurement Technique Development and Other Services, Proa* DoE Annual Review

for Advanced Materials R&D Branch, vail, Colorado, October 24-26, 1978,

pp. 41-50 (January 1979).

Hovel, H. J. , Solar Cells, Semiconductors and Semimetals, r. k. willardson and A. C. Beers, Eds., Vol. 11, p. 47 (Academic Press, New York,

1975).

Ross, R. G. , private communication.

Lehovec, K. , private communication. Preliminary results obtained under

subcontract during present reporting period.

Gardner, W. W., Depletion-Layer Photoeffects in Semiconductors, Phys*

Rev* jm, 84-87 (1959).

Anderson, L. K. , Detection of Microwave Modulated Light, 1964 Int.

Solid-State Circuits Conference Digest of Technical Papers, pp. 60-61.

Sawyer, D. E., A Study on p-n Junction Photodetectors Utilizing Internal Parametric AE5>lification, Sperry Rand Research Report SRRC-PR-65-34,

issued April 1965, Final Report of Work Performed 1 April 1964 - 31

March 1965. Prepared for the Department of the Navy, Bureau of Navail

Weapons, Research Division (RRRE-3), Washington, DC. Contract No. N600

(19) 61770.

Lehovec, K. , and Fedotowsky, A., Degradation of Solar Cell Efficiency by

Sheet Resistance, Solar Energy 21^, 81-86 (1978).

Sawyer, D. E., Kessler, H. K. , and Schafft, H. A., Measurement Techniques for Solar Cells, Quarterly Report, Jemuary 1 to March 31, 1978,

NBSIR 78-1513 (September 1978).

Petian, G. , La Theorie des Functions de Bessel, Centre National de la

Recherche Scientifique, p. 210 (Paris, 1955).

48
APPENDIX A

Laser Scanning of

Active Integrated Circuits and

Discrete Semiconductor Devices'’

D. E. Sawyer, D. W. Berning, and D. C. Lewis**

Institute for Applied Technology

National Bureau of Standards

Washington, D.C.

The device laser scanning work conducted In the Electronic Technology Division of the

National Bureau of Standards (NBS) is described. The scanner constructed at NBS is sketched briefly; this is follow^ by Illustrations demonstrating its usefulness for de- termining on a point-by-point basis the inner workings of active semiconductor devices. The scanner is non-damaging to all devices tried and it has been used to map d.c. and

high-frequency gain variations in transistors, reveal areas of the device operating in a nonlinear manner, electronically map temperatures within devices, determine internal logic states in IC’s and selectively change these states at will. It has also been used to perform

hitherto impossible measurements on flip-chip bonded devices, that is, seeing the circuit

electrical operation and the metallization pattern through the back side of the chip. Applications to other strucures and devices including solar cells are suggesed.

T

he EFFECT OF DIRECTING LIGHT of energy somewhat

greater than that of the band gap on a semiconductor is to create electron-hole pairs within the material which can be collected as a photocurrent. If the light is focused to a spot and moved over exposed semiconductor portions of a device, it is possible to learn a

great deal about the internal operation of the device by interpreting the photoresponse. The interpretation is facilitated if the photoresponse is presented on the screen

of a cathode ray tube whose electron beam is deflected

in synchronism with the moving light spot since a “pic- ture” of the photoresponse of the device is presented.

Interpretation of the display can yield, on a point-bypoint basis, information about the device which can be

used to determine internal characteristics. This probing

technique has applications in device testing, device design, and reliability. Provided that the photon energy of

the light is less than a few times the band gap energy of

the semiconductor one would not expect either short or

long term device degradation to occur due to the light (provided of course that the optical power density at the specimen is not excessive). No device damage due to the

flying-spot scanner has been observed.

Scanner Description

The scanner to be described here is based on previous

work^ * ® and several innovations. A schematic outline of the scanner is shown in Fig. 1; a photograph of the apparatus is shown in Fig. 2.

*This work was conducted as part of the National Bureau of Standards program on Semiconductor Measurement Technology with principal funding from the Defense Advanced

Research Projects Agency through ARPA Order 2397. ••Present address: Office of Naval Research, Washington, DC

22217.

Light for the laser scanner is provided by either of two low power He-Ne cw lasers. The wavelength of one laser is 1.15 /un; the wavelength of the second laser

is 0.633 /un. Mirrors M, and M, are used to fold the output from

the 1.15-/im beam so that the scanner can be made

more compact and rugged; mirror M, is used for the same purpose with the 0.633-;jn beam. Mirror Ms is used to select which laser output is to be used with the scanner; it can be used to block the l.lS-^mi beam and

simultaneously insert the 0.633-/im beam into the scanner, or it can be used to deflect the 0.633-/mi beam out

of the optical circuit of the scanner. The analyzer shown at the output of the 0.633->xm

laser in Fig. 1 is used to control the intensity of the

radiation delivered to the device under test. Since the output of the laser is plane polarized, rotation of the analyzer can vary the transmissivity from 0 to 1. There

is no intensity control on the LlS-^mi laser.

OISPUY SCREEN Fig. I —The light and signal paths of the dual laser scanner.

SOLID STATE TECHNOLOGY/June 1977 , Volume 20 , Number 6 49
Fig. 2—Photograph of the laser scanner.

Fig. 3—Optical photograph of the surface topology for a microwave transistor consisting of four cells.

Mirrors V and H are electrically driven to provide

orthogonal deflections of the light beam. Light from mirror H passes through a microscope and is focused to a spot on the device to be examined. The same electrical signals that drive mirrors V and H also deflect a spot on

a cathode ray display screen in synchronism with the

laser scan. The display screen is on the lower left of Fig. 1. The lens L, between the vertical and horizontal deflection mirrors refocuses the vertical deflection from V

onto the horizontal mirror, H. The beam diverges from

the horizontal deflection mirror, H, to form the scanning

raster. The scan raster typically covers the same field of view that can be seen with the eye when the microscope

is used in its customary manner. Light reflected from the specimen is used to identify the portion of the specimen

being scanned. The reflected-light circuit uses a half- silvered mirror in the microscope which is an integral part of the microscope’s vertical illuminator, a lens L,, and a photocell. Laser radiation reflected from any point on the specimen is directed by the half-silvered mirror onto the lens and is focused to a fixed point on the

photocell. The photocell signal modulates the display screen to present a picture of the device surface topography. Used this way, one could call the apparatus a

“flying-spot microscope.” The primary purpose of the

reflected-light circuit is to permit correlation of the device response with surface features such as metallization areas. This is accomplished simply by mixing to- gether the signals from the scanned specimen and the

photocell. Alternatively, a color display screen has been

used with the photocell signal fed into one color channel and the electrical signal (s) from the scanned speci- men into the other (s). A recent publication* describes

in detail the construction and operation of the entire scanner. Two lasers are used for a greater measurement flexibility. Visible or near-infrared radiation incident on sili- con creates electron-hole pairs with a generation rate which exponentially trails off with distance into the material. The penetration depends on the wavelength of the

incident radiation. The visible light from the 0.633-/im

laser has a characteristic penetration depth of about 3 ^ in silicon.'^ Because most modem silicon devices have their active regions within a few micrometers of

the surface, the 0.633-;nm laser is quite effective in exciting active regions of silicon operating devices. The

intensity at the specimen can be varied to produce junction photocurrents over the range from about 10 pA to about 0.1 mA.

Silicon at room temperature is almost transparent to the 1.15->im infrared radiation from the second laser; the characteristic penetration depth of this radiation

is about 1 cm.® The infrared laser is used for three

classes of measurements: 1) examination of the silicon- header interface; 2) device temperature profiling; and

3) examination of the device through the backside of the silicon chip. Each of these applications makes use of the penetrating nature of the radiation. In the first application, the reflected-light circuit is used to look

through the silicon wafer and observe irregularities at the silicon-header interface in the “flying-spot micro- scope” mode. The second application makes use of the temperature sensitivity of the silicon absorption; a larger

signal is produced on the display screen for those device

portions which are warmer than others. Utilizing this

sensitivity, one has an electronic technique for thermal mapping of devices which appears to have a number of advantages over the more traditional methods.® The

third application uses the penetrating infrared radiation

to photogenerate carriers deep within silicon devices;

this capability allows the operation of devices that are bonded face down (e.g., beam lead devices) to be ex- amined.

Mapping of Device Electrical and Thermal Characteristics Figure 3 shows photographs of the metallization pattern of a type 2N4431 UHF transistor. This widely

available type is designed to furnish 5 W at frequencies up to 1 GHz. There are four in-line cells electrically connected in parallel; these are shown in the left photograph. The total active area of the transistor is a rec- tangle 1.2 mm long and 0.15 mm wide. The emitter

June 1977/SOLID STATE TECHNOLOGY

50
and base fingers are interdigitated with the emitter fing- ers coming in from the right, and the base fingers coming in from the left. The finger metallization is 2 jum in width, and the stripe separation is 8 The metal

stripe separation allows the active device regions to be

accessible to laser irradiation. For the results that will be shown, the devices were scanned while they were

connected and biased in the common-emitter configuration. The signal for the display screen was taken from a 60 ohm resistor which served as the collector load. For

several of the transistors, it was found that regions of high temperature, so-called “hot-spots,” would form

within the acceptable operating range listed in the manufacturer’s data sheet. This observation was used to aid

in understanding the many things the scanner can tell us about the way a device really works. Figure 4 shows

photographs of the^isplay screen. The upper left image

is the infrared response for a collector-emitter voltage

of 26 V and a collector current of 250 mA. The transis- tor is operating just outside of the hot-spot region. Little of the incident optical energy is absorbed in the device

active regions so the display screen signal is weak. The

infrared response was actually quite uniform over the

device, and the apparent nonuniformity captured in this photograph was due to system noise which had the ef- fect of modulating the presentation of the photoresponse.

The lower left photograph was made for the same scan- ning conditions but with the bias adjusted slightly to put

the transistor into hot-spot operation.® The region of enhanced photoresponse, the white area, is the hot-spot

region as confirmed with the use of a passive infrared microscope. The photoresponse is proportional to the number of electron-hole pairs photogenerated in the device active region. For radiation from the infrared laser, which is lightly absorbed at room temperature, the number of pairs photogenerated increases with temperature because the optical absorption coefficient increases with

temperature. This is due primarily to the well-known

change in the silicon bandgap with temperature and an

increase in the phonon population. It is possible to as- sign temperature values to this thermal-enhancement of the 1.15-^m response, and a paper describing the details

is in preparation.

The upper right photograph is for the same hot-spot

conditions, but it was made using the response to the

visible laser. Essentially all of the incident light is ab- sorbed in the active regions at room temperature and so no enhancement in the hot-spot is observed. During normal operation, each laser produces optical radiation at a series of discrete wavelengths centered about the nominal wavelength corresponding to the in- dividual allowed axial modes of the laser. Self-mixing

of these wavelengths modulates the light simultaneously

at several frequencies. The modulation frequencies are

multiples of a fundamental one, and for the particular

visible laser used, the light is self-modulated at 500 MHz

and 1.0 GHz to a degree adequate for determining the response of devices to light modulated at these frequencies. One inserts a radio receiver between the specimen

Fig. 4—Electrical responses of microwave transistor. Top

left: infrared photoresponse outside the hot-spot regime; bottom left: infrared photoresponse in the hot-spot regime;

top right: visible photoresponse in the hot-spot regime; bottom right: visible photoresponse with the modulated

light at 500 MHz.

and the display electronics and tunes the receiver to the

selected frequency.

The lower right photograph in Fig. 4 is for the same

bias and optical wavelength as for the upper right photograph. In this case the radio receiver is used and the image produced is the response of the transistor to the component of the laser light modulated at 500 MHz.

This image is interpreted as the spatial map of the 500- MHz gain of the transistor. It is well known that the hotspot formation can influence high-frequency device

gain.® The hot-spot area is revealed quite dramatically,

and it can be seen that there are actually two hot spots symmetric about a cell boundary.

By scarming a device with an electrical signal applied

to its usual input port, one can map on the display screen the regions of the device which are operating nonlinearly at the signal frequency. All that is required is that an i.f. amplifier and detector, tuned to the differ- ence frequency of the electrical signal and a laser mode

beat, be inserted between the device output and the dis- play screen.® Nonlinearities can indicate stress points

and so this mapping may be useful in predicting certain types of device failures. It may also be useful in selecting

linear devices for applications in which many devices are used cascaded throughout a communications system;

component nonlinearity in such applications is to be

avoided since it may produce cross-talk between channels. For these reasons, this unique ability to map device

nonlinearities should make the laser scanner invaluable

for device design, testing, and reliability.

Internal Operation of Integrated Circuits The results of scanning a type 7438 dual-input NAND

gate IC with 0.633-/im light are shown in Fig. 5. The

schematic of the bipolar IC appears in the upper right

quadrant along with the truth table. The output transis- tor (the output is labeled Y) is normally in its high, or

SOLID STATE TECHNOLOGY/June 1977

51
Fig. 5—Type 7438 dual-input NAND gate. Top right; sche- matic and truth table; top left: reflected-light image; bottom

left: photoresponse superimposed on reflected-light image

with output in high state; bottom right: photoresponse

superimposed on reflected-light image with output in low

state.

Fig. 6—Photograph of the display when scanning through

the back side of a silicon-on-sapphire C-MOS device.

nonconducting state, and to switch it into its low state,

i.e., into saturation, requires that both inputs A and B be

in their high states. The overall NAND gate operation

is specified by the trio of numbers representing A, B, and Y, in that order. As an example, the case in which

A and B are high to cause the output to go low is listed in the table as 110. The top left photograph was generated solely by the scanner’s reflected light circuit and

it shows the metallization. The output transistor occupies the upper right quadrant. To reveal the NAND

gate’s internal operation, it was biased for normal ser- vice with a 330 ohm resistor inserted in the supply bus, and the voltage variation across this resistor during laser scanning modulated the display screen. A transistor operating with significant small-signal current gain will show a large photoresponse and consequently produce a

bright image on the display screen. On the other hand,

a transistor operating with little small-signal current

gain, such as when it is in saturation, will produce al- most no image. Thus, from the photoresponse of the elements in the circuit, the different logic states can be

deduced. Naturally, changing the state of the NAND

circuit yields a new photoresponse display of the active elements.

The photograph in the lower left quadrant of Fig. 5

is the superposition of the scanned IC’s electrical re- sponse and the reflected-light metallization signal for the 001, 101, and 011 states; all give the same results. The

output transistor is in its high state, and an appreciable

photoresponse is obtained from portions of the basecollector junction not covered by metallization. The input

transistor is located directly below the output transistor, and very little photoresponse is observed from it because no appreciable amplification of the photocurrent generated in the input transistor can occur unless the transistor is out of saturation, i.e., unless both inputs are in

their high states. For the photograph in the lower right quadrant of

Fig. 5, the circuit state is 110, and the output transistor

is in saturation with little photoresponse obtained from

it. The input transistor is out of saturation, and the two L-shaped areas, one under another, below the out- put transistor are the unobstructed base-collector junction regions around the emitters. The illuminated re- gion to the left is the inter-stage transistor, and the hori- zontal bar below this is the 4K ohm load for the input

transistor. MOS ICs also have been inspected with the laser scanner. In addition to observing the states of all the

logic elements, and the states of the input-output cir- cuits in a shift register, it has been possible to change

the state of a desired logic element from 0 to a 1 or vice versa nondestructively with the laser spot.'® This ability could have far-reaching implications for the testing of LSI circuits, particularly for testing those portions that are embedded, i.e., with no direct connections to external leads.

Examining IC's Through the Wafer

In the work described so far, the scanning light beam

was incident on the side of the chip containing the active circuit. But, it is possible to scan a device using

penetrating light such that the light enters the back

surface and passes almost entirely through the chip before being partially absorbed in the device active region or reflected by the metallization. The 1.15-^ He-Ne

laser wavelength is nearly optimum for accomplishing

this in silicon devices. Because of the penetration, photocurrents from deep within the chip may contribute to the overall response. Light scattering due to back surface irregularities must be minimized in order to maintain resolution, and

this requires cither that the back side be smooth, or that a drop of liquid be placed on the surface having an

index of refraction approximately equal to that of the semiconductor. Back surface scanning was successfully tried on three

different types of devices; C-MOS on sapphire, bulk C- MOS, and a bulk bipolar circuit. The C-MOS on sapphire circuit, a type 4007, was

scanned from the back side using visible light (X =

0.633 pm) because sapphire is optically transparent. A

drop of light oil was deposited on the back of the deJune 1977/SOLID STATE TECHNOLOGY

52
vice substrate to provide an optically smooth surface. The photograph of the scanner display shown in Fig. 6 was obtained using the reflected li^t signal mixed with

the photoresponse of the inverter. Of the six transistors shown in this figure, only the lower right hand one was

electrically connected. The five brightly lit stripes are

the gate area of the electrically “on” device. The bulk silicon C-MOS integrated circuit was also a 4007; however, since the substrate was silicon, it was

necessary to use the 1.15-jam wavelength. Figure 7 shows the reflected light image. No oil was needed as the back side was a polished surface. The third device was a beam-leaded bulk silicon bi- polar 709 operational amplifier. In this case oil was

deposited on the back of the device die to provide an

optically smooth surface. Figure 8a shows the 1.15-/im

scanning image obtained when the amplifier was connected to provide a gain of 10 and with 0 V input. The

reflected-light signal is superimposed on the photoresponse. Figure 8b shows the device for the same conditions except that the input voltage is now —1.5V, a

value sufficiently large to saturate the 709. These three examples demonstrate a capability of the scanner which can be very useful when metallization on the top of the chip obscures the underlying device

active regions but these regions are accessible from the back, or when it is desired to inspect devices bonded

into frames or packages “face down” with direct access

to the active surfaces blocked by mounting materials.

Application to Photodetectors and Solar Cells The laser scanner is an obvious candidate for analyzing photodetector operation. Devices made from a niun- ber of materials, including silicon and indium arsenide,

and designed for optical communication, have been

scanned with unmodulated light, and light modulated at frequencies up to 1.2 GHz, to study gain and spatial response variations. These devices have been both those

that utilized internal (avalanche) gain, as well as those which utilized simple photodetection. Even vacuum devices have been scanned; measurements have been made

of the uniformity of cathode response for a 931-A photomultiplier to 0.633-/un light modulated at 120 MHz.

Experimental solar cells designed for space applications

have been laser scanned and the results compared with

those obtained with a scanning electron microscope in the electron-beam-induced-current (EBIC) mode. The

defects located with the SEM were also found with the

laser scanner, in these preliminary experiments. The

scanner described in this paper should also be useful in observing variatfons in solar cell response with light

level, and this could be invaluable for cell design. This could be done by scanning the cell with a “small sig- nal” laser while at the same time exposing the cell uni- formly to an auxiliary source which would be varied

step-wise in intensity.*^

Summary

The scanner work described makes use of the fact

SOLID STATE TECHNOLOGY/June 1977

Fig. 7—Photograph of the display when scanning through

the back side of a bulk silicon C-MOS device with the infra- red laser.

Fig. 8—Images obtained when scanning a commercial flip- chip operational amplifier with the infrared laser, (a) In- put at 0 V. (b) Input at —1.5 V.

that semiconducting materials are photosensitive as part

of their basic nature. Thus, semiconductor devices such

as diodes, transistors, and entire integrated circuits made

from these materials can be studied using optical radiation. The effect of the optical radiation is to generate

electron-hole pairs in a non-damaging way within the specimen. These current-carriers can stimulate device behavior by taking the place of signal current-carriers which are normally supplied by leads fixed to the device. In contrast to signals applied via the leads, which are

fixed in position, the optical excitation can be moved

53
over the surface and within the., bulk, and the response

of the structures can be studied on a point-by-point

basis to learn the inner workings of the device. In several cases, the scanner has been used to observe

device phenomena which one has not been able to study

before. The scanner can map the flow of logical information in MOS and bipolar IC’s. It can map the operation

of transistors at ultrahigh frequencies; it can electronically map the temperature distribution; it can pinpoint

the portions of operating devices which are operating in a non-linear manner; and it has the unique ability to look through the back side of silicon chips to observe

circuit operation.

References

1. Potter, C. N., and Sawyer, D. E., “A Flying-Spot Scanner,” Rev. Sci. Instrum, vol. 39, 180-183 (1968).

2. McMahon, R. E., “Laser Tests IC’s with Light Touch,”

Electronics, vol. 44, 92-95, (April 12, 1971).

3. Beiser, L., "Laser Beam Scan Enhancement through Peri- odic Aperture Transfer,” Applied Optics, vol. 7, 647-650

(1968).

4. Sawyer, D. E., and Berning, D. W., “A Laser Scanner for Semiconductor Devices,” NBS Special Publication 400-24, February 1977. Available from the Superintendent of Documents, U.S. Govt. Printing Office, SD Cat. No. C13.10:

400-24.

5. Dash, W. C., and Newman, R., “Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77° and

300°K,” Phys. Rev., vol. 99, 1151-1155 (1955).

6. Sawyer, D. E., and Berning, D. W., “Thermal Mapping of

Transistors with a Laser Scanner,” Proc. IEEE, vol. 64, 1634-1635 (1976).

7. Sawyer, D. E., and Lanyon, H. P. D., “Determining Sili- con Device Temperatures By Measuring the Thermally-En- hanced 1.15 /im Photoresponse,” to be published.

8. Oettinger, F. F., and Rubin, S., "The Use of Current Gain

as an Indicator for the Formation of Hot Spots Due to Current Crowding in Power Transistors,” IEEE 10th Annual Proceedings, Reliability Physics, 12-18 (1972).

9. Sawyer, D. E., and Berning, D. W., “Mapping Nonlinearities Over the Active Regions of Semiconductor Devices,”

Proc. IEEE, vol. 64, 1635-1637 (1976).

10. Sawyer, D. E., and Berning, D. W., "Laser Scanning of MOS IC’s Reveab Internal Logic States Non-destructively,” Proc. IEEE, vol. 64, 393-394 (1976).

11. Lanyon, H. P. D., Worcester Polytechnic Institute, Worcester, MA, private communication.

June 1977/SOLID STATE TECHNOLOGY
APPENDIX B

STABILITY OF (THIN FILM) SOLAR CELLS AND MATERIALS

WORKSHOP PROGRAM

Monday, May 1, 1978

8:30 a.m. REGISTRATION

9:15 a.m. WELCOME, PERSPECTIVE, AND WORKSHOP OVERVIEW

9:45 a.m. SESSION I: Status of Present Reliability Testing, Failure Modes, Failure

Mechanisms, and Data for Advanced-Cell Materials

Chairman: A. Barnett (Institute for Energy Conversion, U. of Delaware,

Newark, DE)

[CdZn]S/Cu2S

J. D. Meakin and J. E. Phillips (Institute for Energy Conversion, U. of

Delaware, Newark, DE)

CdS/Cu-TernaHes

L. Kazmerski (Solar Energy Research Institute, Golden, CO)

PolycryBtalline Si

T. L. Chu (Southern Methodist U., Dallas, TX)

COFFEE

MIS and Conducting Oxide/Si

W. Anderson (Rutgers U., Piscataway, NJ)

Conducting Oxide/Si Heterojunctiona

R. L. Anderson (Syracuse U., Syracuse, NY)

MIS

S. J. Fonash (Pennsylvania State U. , University Park, PA)

1:00 p.m. LUNCH

2:00 p.m. SESSION I (Continued)

AMOS GaAa

R. Stirn (Jet Propulsion Laboratory, Pasadena, CA)

Polycryetalline GoAa

S. Chu (Southern Methodist U., Dallas, TX)

Amox^houa Si

D. Carlson (RCA Corp., Princeton, NJ)

COFFEE

55
SESSION II: Measurements and Tests used to Define Stability in Related

Technologies

Chairman: R. I. Scace (National Bureau of Standards, Washington, DC)

Silicon Cell Space Program Experience

P. lies (Optical Coating Laboratory, Inc., City of Industry, CA)

Concentrator Solar Celle

W. McLevige (Sandia Laboratories, Albuquerque, NM)

5:00 p.m. ADJOURN

6:30 p.m. CASH BAR (Ramada Inn)

7:30 p.m. WORKSHOP DINNER (Ramada Inn)

Tuesday, May 2, 1978

9:15 a.m. SESSION II (Continued)

DoD Experience

J. Adolphson (NASA Goddard Space Flight Center, Greenbelt, MD)

Methods and Teste for Devices at the Bell Telephone Laboratory

C. W. Green (Bell Telephone Laboratories, Allentown, PA)

Power and CMOS Devices

S. Kukumaris (RCA Corp., Somerville, NJ)

COFFEE

Interdiffusion Phenomena

L. Kazmerski (Solar Energy Research Institute, Golden, CO)

Corrosion

R. Frankenthal (Bell Telephone Laboratories, Murray Hill, NJ)

Terrestrial Silicon Array Field and Test Eoi:perienae

R. Ross (Jet Propulsion Laboratory, Pasadena, CA)

1:00 p.m. LUNCH

2:00 p.m. SESSION II (Continued)

Designing Accelerated Aging Teste for Predicting Array Life

D. Carmichael (Battelle Columbus Laboratories, Columbus, OH)

56
I

!

2:30 p.m. SESSION III: Working Group Sessions — To Identify tests and measurement

procedures that can be used to enhance the prediction of

material and device stability for each of the three solar

cell groups.

GROUP 1: [CdZn]S/Cu 2 S, CdS/Cu-Ternarles, CdS/InP, and Amorphous SI Chairman: L. Kazmerski (Solar Energy Research Institute, Golden, CO)

Location: Green Auditorium

GROUP 2: Polycrystal line SI, MIS, and Conducting Oxide SI Chairman: J. Shewchum (McMaster U., Hamilton, Ontario)

Location: Lecture Room A

GROUP 3: Polycrystal line and AMOS GaAs

Chairman: R. Stirn (Jet Propulsion Laboratory, Pasadena, CA)

Location: Lecture Room D (n.b.: Wednesday, Group meets In Employee's Lounge)

5:00 p.m. ADJOURN

Wednesday, May 3, 1978

9:15 a.m. SESSION III (Continued)

GROUP 1: [CdZn]S/Cu 2 S, CdS/Cu-Ternarles, CdS/InP, and Amorphous SI Chairman: L. Kazmerski (Solar Energy Research Institute, Golden, CO)

Location: Green Auditorium

GROUP 2: Polycrystal line SI, MIS, and Conducting Oxide SI Chairman: J. Shewchum (McMaster U., Hamilton, Ontario)

Location: Lecture Room A

GROUP 3: Polycrystal line and AMOS GaAs

Chairman: R. Stirn (Jet Propulsion Laboratory, Pasadena, CA)

Location: Employee's Lounge

10:50 a.m. COFFEE

11:20 a.m. TOUR (Selected Energy-Related Projects of NBS)

Tour ia for all partioipanta not involved in the preparation of working

group eurrmviriea for presentation in Session IV.

1:00p.m. LUNCH

1

57
2:00 p.m. SESSION IV: Working Group Presentations of Summaries and Recommendations

Chairman: H. A. Schafft (National Bureau of Standards, Washington, DC)

3:30 p.m. SESSION V: Closing Remarks

Chairman: D. E. Sawyer (National Bureau of Standards, Washington, DC)

4:00 p.m. ADJOURN

CREDITS;

Workshop Sponsor:

Advanced Materials R&D Branch

Division of Solar Technology

Department of Energy

Steering Committee:

Allen Barnett

University of Delaware

Gordon Gross

Solar Energy Research Instltu

Gary Jones

Sandla Laboratories

J. W. Lathrop

Clemson University

L. M. Magld

Department of Energy

Joseph M. Morablto

Bell Telephone Laboratories

Ronald Ross

Jet Propulsion Laboratory

R. J. Stlrn

Jet Propulsion Laboratory

Brown Williams

David Sarnoff Laboratories

Organizing Committee:

Donald L. Feucht

Department of Energy

David E. Sawyer

National Bureau of Standards

Harry A. Schafft

National Bureau of Standards

NBS Workshop Committee:

Chairman - David E. Sawyer

Coordination - Harry A. Schafft

Registration - Elaine C. Cohen

Arrangements - Sara R. Torrance, JoAnn Lorden, Elaine C. Cohen

58
APPENDIX C

PHOTOVOLTAICS MATERIAL AND DEVICE MEASUREMENTS WORKSHOP

Preliminary Plans

Dates June 11-13, 1979

Location Stouffer's National Center Hotel

2399 Jefferson Davis Highway, Arlington, VA 22202

Sponsor Solar Energy Research Institute in cooperation with NBS

Purpose

To accelerate the development of thin-film solar cells by improving the

versatility and relied>ility of material and device measurement techniques.

Objectives

1. Identify and discuss the techniques which have been used to measure

selected key properties of thin-film polycrystalline materials and

devices.

2. Discuss the applicability of these techniques to the understanding of

thin-film polycrystalline materials and devices.

3. Identify measurement techniques which require improvements.

4. Identify new measurement techniques.

5. Suggest methods for achieving:

more meaningful materiaLl and device measurements

** greater measurement reproducibility

® greater efficiency in the develojanent and fabrication of

thin-film devices

® greater confidence in con^aring and evaluating thin-film device

measurements

.

6. Foster greater communication between device researchers and measurement

methods experts.

59
December 21, 1978

Program

The workshop will have three sessions devoted to formal presentations, a general discussion session, and eui evening poster session for informal

presentations. David Sawyer of NBS, and Joseph M. Morabito of Bell Labs,

will serve as co-chairmen for the workshop.

The sessions will be designed to achieve the objectives stated above;

however, thin-film amorphous materials and devices will not be included in

this workshop.

Each session will be organized and conducted by a set of co-chairmen. The

following have agreed to serve as session co-chairmen:

Structural/Chemical Session

John D. Meakin, University of Delaware

Fritz Wald, Mobil -Tyco

Optical/Electro-Se s sion

Lewis M. Frass, Chevron Research Company

Alan Fahrenbruch, Stanford University

Charge Transport Session

P. Daniel Dapkus, Rockwell International

H. H. Weider, Naval Ocean Systems Center

Poster Session

L. L. Kamerski, SERI

Steve Hogan, SERI

Discussion Session

David Sawyer, NBS

Joseph M. Morabito, Bell Laboratories

Each session will have one invited keynote speaker whose presentation will be

scheduled for 45 minutes (35-minute presentation; 10-minute question and answer period). This will be followed by several invited contributed papers

which are scheduled for 40 miutes (20-minute presentation; 10-minute question

and answer period). Papers solicited through a call for papers will be used

to fill in each session or be scheduled for the poster session.

Documentation

Foxir weeks prior to the workshop ^LL1 presenters will be required to submit an extended abstract (two-page maximum) . The abstracts will be compiled,

printed and distributed at the meeting. Specific requirements for the

content of the abstracts will be defined by the co-chairmen for each

session.

Following the workshop a proceedings will be mciiled to all registrants and

distributed through TIC and NTIS. The proceedings will include the extended

abstracts, viewgraphs, and written questions emd answers from each

presentation.

60
APPENDIX D

Publications and Talks

0.1 Recent Publications:

Sawyer/ D. E./ A Technique for Using An Optical Scanner to Reveal Solar

Cell Defects, Pvoc. 13th IEEE Photovoltaic Specialists Conf ,, Washington, D.C., June 5-8, 1978, pp. 1249-1250 (1978).

Sawyer, D. E., Kessler, H. K. , and Schafft, H. A., Measurement

Techniques for Solar Cells, Quarterly Report, September 15 to December

31, 1977, NBSIR 78-1488 (July 1978).

Sawyer, D. E., Kessler, H. K. , amd Schafft, H. A., Measurement

Techniques for Solar Cells, Quarterly Report, January 1 to March 31,

1978, NBSIR 78-1513 (September 1978).

Sawyer, D. E., Kessler, H. K. , and Schafft, H. A., Solar Cell

Measurement Technique Development and Other Services, Pvoc» DoE Annual

Review for Advanced Materials RSD Branch, Vall, Colorado, October 24-26,

1978, pp. 41-50 (Janueury 1979).

D.2 Publications In Press

Sawyer, D. E., Kessler, H. K. , and Schafft, H. A., Measurement Techniques for Solar Cells, Quarterly Report, April 1 to June 30, 1978,

NBSIR 78-1488 (July 1978).

Semiconductor Measurement Technology: NBS/DOE Wbrkshop, Stability of

(Thin Film) Solar Cells and Materials, D. E. Sawyer and H. A. Schafft,

Editors, NBS Sipeclal Publication 400-58 (August 1979).

D.3 Talks .

Sawyer, D. E., A Technique for Using An Optical Scanner to Reveal Solar

Cell Defects, Proc. 13th IEEE Photovoltaic Specialists Conf., Washlng1 ton, D.C., June 5-8, 1978.

l

I Sawyer, D. E., Kessler, H. K. , and Schafft, H. A., Solar Cell Measurement Technique Development and Other Services, Proc. DoE Annual Review

for Advanced Materials R&D Branch, Vall, Colorado, October 24-26, 1978.

Sawyer, D. E., Solar Cell Laser Scanning, Invited talk and seminar. Institute of Energy Conversion, University of Delaware, November 10,

1978.

1

I

61
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. N6SIR 60-2027

Z, $ Acc«$sloa No,

4. TITLE AND SUBTITLE

Measurement Techniques for Solar Cells: Annual Report for

the Period September 15, 1977 to December 14, 1978

5. Publication Date July 1930

7. AUTHOR(S)

D. E. Sawyer, H. K. Kessler, and H. A. Schafft

8. Performing Organ. Report No. I

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

tft.

11. Contract/Grant No. Task Order A05^"SE

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS csfreef. City, state, ZIP)

Advanced Materials RSD Branch

Division of Distributed Solar Technology

Department of Energy

20 Massachusetts Ave., N.W. , Washington, D. C. 205^5

13. Type of Report & Period Covered Interim Report/Sep. 15,

1377-Dec. 15, 1978

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached. i I

16. ABSTRACT (A 200^word or teas factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.)

Ilils is a repot I ol work I it a proyraiii on solar coll mcasurcnion L lcchiilc|iie Jcvelopmctil and olliei services which

was performed at NBS in September 15, 1977 to December Hi, 1978. The objectives of the program are to assist

the DOF thin film photovoltaic effort by developing solar cell device and materi.al measurement techniques using

llic NUj-deve lotted lasitr flylny-spoL scanner, by assisting DOE In orynniziny and hosting appropriate wot ksliops

and symposia, and by providing general consultation and liaison service.

A technique was developed which employs forward biasing during laser scanning of the solar cell to reveal defects

including cracks, metallization breaks, and regions of metallization not in ohmic contact with the underlying

semiconductor. The technique also has provided information useful to cell designers. The experimental results

obtained are consistent with predictions made by a first-order theory developed in-house, and by subsequent

iii.Uhfinal 1 ( ii 1 m(»df'llnq perfoinir'd by the University of Sotilhern California uiiih'r snhconlract. Single-crystal sill*

con artifacts have been designed to couple laser scanning measurements with mathematical analyses to define

quantatively the capabilities of the laser scanner techniques to detect cell defects and spatial nonun i format i es of cell performance.

The laser scanner was designed originally to analyze Integrated circuit devices. A number of scanner modifications were made and ancillary apparatus was developed to adapt the scanner for examining solar cells.

These modifications include an increase in the specimen area that can be scanned without specimen repositioning,

the use of a more efficient means of electronically coupling scanned cells to the display electronics, and the construction of a hlgh-intens i ty source for light-biasing cells during scanning.

A workshop on Stability of (Thin Film) Solar Cells and Materials was held May 1-3, 1978 at NBS, Gaithersburg,

MD. Plans were Initiated for a workshop on photovoltaic material and device measurements scheduled to be held

In the Washington, 0. C. area in the spring of 1979.

17, KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first tetter of the first key word unless a proper name;

separated by semicolons)

Device measurements; laser scanning; light-biasing; metallization; ohmic contacts;

reliability; semiconductor measurements; sheet resistance; solar cell; solar cell measu remen ts; solar Cell Stability.

18. AVAILABILITY Unlimited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

fxl Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

68

22. Price

$7.00

USCOMM- DC